:iw*r$itg 


Name  of  Book  and  Volume, 


Division 
Range 

.Shelf. 

Received  .f 


187^ 


University  of  California; 

'V 


GIFT   OF 


187* 


I 


ELEMENTS 


OF 


PHYSICS, 

OR 

NATURAL    PHILOSOPHY, 

GENERAL  AND  MEDICAL, 

EXPLAINED  UfDEPEITDEJfTIT  OF 

TECHNICAL  MATHEMATICS. 


IN  TWO  VOLUMES. 
VOJL.  II. — PART  I. 

COMPREHENDING  THE  SUBJECTS  OF 

HEAT  AND  LIGHT. 


BY  NEIL  ARNOTT,  M.  D., 


OF  THE  HOYAL  COLLEGE  OP  PHYSICIANS. 


FIRST  AMERICAN  FROM  THE  FIRST  LONDON  EDITION. 


CAREY   &   L.EA. 

1831. 


ADVERTISEMENT 

f     '  ^PT  /  r    uf 


TO 


VOL.  II.— PART  I. 


THE  second  part  of  volume  ii.,  comprising  the  sub- 
jects of  ELECTRICITY,  MAGNETISM,  and  ASTRONOMY,  and 
concluding  the  work,  will  be  put  to  press  after  the  pub- 
lication of  the  present  part.  The  first  volume  was 
originally  published  without  the  second,  although  the 
whole  manuscript  was  prepared,  because  other  works 
on  Natural  Philosophy  were  offered  to  public  notice 
about  the  time.  The  delay  of  two  years  with  respect 
to  the  second  volume  has  occurred,  because  the  au- 
thor's very  little  leisure  from  the  duties  of  his  profes- 
sion (than  which  perhaps  none  more  interestingly  ab- 
sorbs the  time  and  faculties)  was  completely  taken  up 
by  attending  to  the  repeated  calls  for  editions  of  vol. 
i.  A  friend,  however,  has  superintended  the  printing 
of  the  last  edition,  and  has  allowed  him  to  proceed 
with  vol.  ii.  These  explanations  are  given  as  an  apo- 
logy to  the  many  persons  who  have  honoured  the  work 
by  expressing  disappointment  at  the  tardy  appearance 
of  vol.  ii. 

The  author,  while  preparing  the  fourth  edition  of 
vol.  i.,  received  a  copy  of  a  French  edition,  in  which 
the  translator,  M.  Richard,  to  fit  the  work  for  the  ge- 
neral use  of  public  schools  and  colleges  in  France,  had 


iv  ADVERTISEMENT. 

given  in  notes  the  common  algebraical  formulae  for  the 
various  cases  described.  The  author,  at  one  time,  in- 
tended to  have  done  this  himself,  but  afterwards  deter- 
mined only  to  add  a  few  remarks  on  the  subject  at  the 
end  of  vol.  ii.  To  this  determination  he  still  adheres. 
In  one  of  the  North  American  English  editions  of  the 
work,  there  are  also  copious  notes,  but  as  the  author 
has  not  yet  been  able  to  procure  a  copy,  he  cannot  re- 
mark upon  them. 

In  the  present  or  fourth  edition  of  vol.  i.,  the  subject 
of  speech  is  still  farther  analyzed,  by  a  complete  ex- 
planation of  the  hitherto  unknown  nature  of  the  defect 
called  stuttering  or  stammering;  and  the  discovery  of  its 
nature  has  suggested  to  the  author  an  effectual  reme- 
dy, so  simple,  that  sufferers  in  general  will  be  able  at 
once  to  adopt  it  from  the  description  now  given. — 
That  the  purchasers  of  former  editions  may  not  be 
obliged  to  procure  the  last  on  this  account  alone,  the 
chief  additions  to  the  section  in  vol.  i.  are  here  sub- 
joined. They  occur  at  page  610  of  the  fourth  edition; 
and  they  should  be  inserted  at  page  565  of  the  first 
edition,  at  page  589  of  the  second,  and  at  page  495  of 
the  first  American  edition. 

London,  November,  1829. 


APPENDIX 

TO  EARLY  EDITIONS. 


"  The  most  common  case  of  stuttering,  however,  is 
not,  as  has  been  almost  universally  believed,  where  the 
individual  has  a  difficulty  in  respect  to  some  particular 
letter  or  articulation,  by  the  disobedience,  to  the  will  or 
power  of  association,  of  the  parts  of  the  mouth  which 
should  form  it,  but  where  the  spasmodic  interruption 
occurs  altogether  behind  or  beyond  the  mouth,  viz.  in 
the  glottis,  so  as  to  affect  all  the  articulations  equally. 
To  a  person  ignorant  of  anatomy,  and  therefore  know- 
ing not  what  or  where  the  glottis  is,  it  may  be  sufficient 
explanation  to  say,  that  it  is  the  slit  or  narrow  opening 
at  the  top  of  the  wind  pipe,  by  which  the  air  passes  to 
and  from  the  lungs — being  situated  just  behind  the 
root  of  the  tongue.  It  is  that  which  is  felt  to  close 
suddenly  in  hiccup,  arresting  the  ingress  of  air,  and 
that  which  closes,  to  prevent  the  egress  of  air  from  the 
chest  of  a  person  lifting  a  heavy  weight  or  making  any 
straining  exertion;  it  is  that  also,  by  the  repeated  shut- 
ting of  which,  a  person  divides  the  sound  in  pronouncing 
several  times,  in  distinct  and  rapid  succession,  any  vow- 
el, as  o,  o,  o,  o.  Now,  the  glottis,  during  common 
speech  need  never  be  closed,  and  a  stutterer  is  instant- 
ly cured  if,  by  having  his  attention  properly  directed  to 
it,  he  can  keep  it  open.  Had  the  edges  or  thin  lips  of 
the  glottis  been  visible,  like  the  external  lips  of  the 
mouth,  the  nature  of  stuttering  would  not  so  long  have 


yi  ON  STUTTERING. 

remained  a  mystery,  and  the  effort  necessary  to  the 
cure  would  have  forced  itself  upon  the  attention  of  the 
most  careless  observer;  but  because  hidden,  and  pro- 
fessional men  had  not  detected  in  how  far  they  were 
concerned,  and  the  patient  himself  had  only  a  vague 
feeling  of  some  difficulty,  which,  after  straining,  gri- 
mace, gesticulation,  and  sometimes  almost  general 
convulsion  of  the  body,  gave  way,  the  uncertainty 
with  respect  to  the  subject  has  remained.  Even  many 
persons  who  by  attention  and  much  labour  had  over- 
come the  defect  in  themselves,  as  Demosthenes  did, 
have  not  been  able  to  describe  to  others  the  nature  of 
their  efforts,  so  as  to  ensure  imitation:  and  the  author 
doubts  much  whether  the  quacks  who  have  succeeded 
in  relieving  many  cases,  but  in  many  also  have  failed, 
or  have  given  only  temporary  relief,  really  understood 
what  precise  end  in  the  action  of  the  organs  their  im- 
perfect directions  were  accomplishing. 

'5<-Now,  a  stutterer,  understanding  of  anatomy  only 
what  is  stated  above,  will  comprehend  what  he  is  to 
aim  at,  by  being  farther  told,  that  when  any  sound  is 
continuing,  as  when  he  is  humming  a  single  note  or  a 
tune,  the  glottis  is  necessarily  open,  and  therefore,  that 
when  he  chooses  to  begin  pronouncing  or  droning  any 
simple  sound,  as  the  e  of  the  English  word  berry  (to  do 
which  at  once  no  stutterer  has  difficulty)  he  thereby 
opens  the  glottis,  and  renders  the  pronunciation  of  any 
other  sound  easy.  If  then,  in  speaking  or  reading,  he 
joins  his  words  together,  as  if  each  phrase  formed  but 
one  long  word,  or  nearly  as  a  person  joins  them  in 
singing,  (and  this  may  be  done  without  its  being  at  all 
noted  as  a  peculiarity  of  speech,  for  all  persons  do  it 
more  or  less  in  their  ordinary  conversation,)  the  voice 
never  stops,  the  glottis  never  closes,  and  there  is  of 


APPENDIX  TO  EARLY  EDITIONS.  Vll 

course  no  stutter.  The  author  has  given  thitf  explana- 
tion or  lesson,  with  an  example  to  a  person,  who  be- 
fore would  have  required  half  an  hour  to  read  a  page, 
but  who  immediately  afterwards  read  it  almost  as 
smoothly  as  was  possible  for  any  one  to  do;  and  who 
then,  on  transferring  the  lesson  to  the  speech,  by  con- 
tinued practice  and  attention,  obtained  the  same  faci- 
lity with  respect  to  it.  There  are  many  persons  not 
accounted  peculiar  in  their  speech,  who,  in  seeking 
words  to  express  themselves,  often  rest  long  between 
them  on  the  simple  sound  of  e  mentioned  above,  say- 
ing, for  instance,  hesitatingly,  "el  e think  e you 

may," — the  sound  never  ceasing  until  the  end  of  the 
phrase,  however  long  the  person  may  require  to  pro- 
nounce it.  Now,  a  stutterer,  who  to  open  his  glottis 
at  the  beginning  of  a  phrase,  or  to  open  it  in  the  mid- 
dle after  any  interruption,  uses  such  a  sound,  would 
not  even  at  first  be  more  remarkable  than  a  drawling 
speaker,  and  he  would  only  require  to  drawl  for  a  little 
while,  until  practice  facilitated  his  command  of  the 
other  sounds.  Although  producing  the  simple  sound 
which  we  call  the  c  of  berry,  or  of  the  French  words  de 
or  que,  is  a  means  of  opening  the  glottis,  which  by  stut- 
terers is  found  very  generally  to  answer,  there  are  many 
cases  in  which  other  means  are  more  suitable,  as  the 
intelligent  preceptor  soon  discovers.  Were  it  possible 
to  divide  the  nerves  of  the  muscles  which  close  the 
glottis,  without  at  the  same  time  destroying  the  facul- 
ty of  producing  voice,  such  an  operation  would  be  the 
most  immediate  and  certain  cure  of  stuttering;  and 
the  loss  of  the  faculty  of  closing  the  glottis  would  be 
of  no  moment. 

"  The  view  given  above  of  the  nature  of  stuttering 
and  its  cure,  explains  the  following  facts,  which  to 


Vili  ON  STUTTERING. 

many  persons  have  hitherto  appeared  extraordinary. 
Stutterers  often  can  sing  well,  and  without  the  least  in- 
terruption,— for  the  tune  being  continued,  the  glottis 
does  not  close.    Many  stutterers  also  can  read  poetry 
well,  or  any  declamatory  composition,  in  which  the 
uninterrupted  tone  is  almost  as  remarkable  as  in  sing- 
ing.   The  cause  of  stuttering  being  so  simple  as  above 
described,  one  rule  given  and  explained,  may,  in  cer- 
tain cases,  instantly  cure  the  defect,  however  aggra- 
vated, as  has  been  observed  in  not  a  few  instances; — 
and  this  explains  also  why  an  ignorant  pretender  may 
occasionally  succeed  in  curing,  by  giving  a  rule  of 
which  he  knows  not  the  reason,  and  which  he  cannot 
modify  to  the  peculiarities  of  other  cases.     The  same 
view  of  the  subject  explains  why  the  speech  of  a  stut- 
terer has  been  correctly  compared  to  the  escape  of  li- 
quid from  a  bottle  with  a  long  narrow  neck,  coming — 
"  either  as  a  hurried  gush  or  not  at  all:"  for  when  the 
glottis  is  once  opened,  and  the  stutterer  feels  that  he 
has  the  power  of  utterance,  he  is  glad  to  hurry  out  as 
many  words  as  he  can,  before  the  interruption  again 
occurs. 

"  Should  the  author's  future  experience  enable  him 
to  simplify  or  render  more  complete  the  views  of  the  na- 
ture and  cure  of  stuttering,  which  he  has  given  above, 
so  as  to  facilitate  the  cure  in  every  variety  of  case,  he 
will  not  fail  to  publish  his  remarks." 


ELEMENTS 


OF 


NATURAL   PHILOSOPHY, 


PART  FOURTH. 

DOCTRINES  OP  IMPONDERABLE  SUBSTANCE    UNDER    THE    HEADS 
OF  HEAT,  LIGHT,  ELECTRICITY,  AND  MAGNETISM. 

To  minds  beginning  this  study,  it  may  facilitate  the  concep- 
tion of  a  substance  which  is  without  weight,  or  at  least  is  im- 
ponderable by  human  art,  to  consider  the  nature  of  air.  Until 
lately  men  were  so  imperfectly  acquainted  with  the  constitu- 
tion of  the  universe  around  them,  that  a  person  placed  in  an 
apartment  which  offered  to  view  nothing  but  the  naked  walls, 
would  have  said  that  it  was  empty,  meaning  literally  what  he 
said;  and  even  when  advertised  that  there  was  air  in  the  room, 
he  would  still  have  been  far  from  possessing  a  clear  notion  that 
it  was  full  of  aerial  fluid  just  as  an  open  vessel  immersed  in  the 
sea  is  full  of  water,  and  that  if  air  were  not  allowed  to  escape 
from  it,  even  so  small  a  body  as  an  apple  could  not  be  pressed 
into  it  additionally  by  less  force  than  fifty  or  sixty  pounds. 
This  truth  however  is  now  clearly  understood,  and  daily  ex- 
emplified in  easy  pneumatic  experiments,  and  in  no  way  more 
strikingly  than  by  the  recent  adoption  of  the  substance  of  air 
in  place  of  feathers,  as  stuffing  for  beds  and  pillows.  An  air- 
tight bag  or  sack  suspended  by  its  lip  in  the  air,  and  held  quite 
open  by  a  hoop  near  its  mouth,  would  appear  empty,  but  if 
then  firmly  closed  above  the  hoop,  it  would  have  imprisoned 
its  fill  of  air,  just  as  a  bag  similarly  managed  under  water  would 
imprison  its  fill  of  water; — and  while  in  some  respects  the  air 
would  be  softer  and  locally  more  yielding  than  feathers,  its  en- 
tire mass  would  be  much  less  compressible.  Now  this  air, 


10  IMPONDERABLE 

when  weighed  by  means  which  modern  science  has  furnished, 
is  found  in  a  cubic  foot  to  contain  somewhat  more  than  an 
ounce,  and  by  strongly  pressing  it,  or  by  causing  it  to  combine 
chemically  with  some  other  substance,  we  can  reduce  it  to  very 
small  bulk,  either  with  the  form  of  a  liquid  or  of  a  solid: 
proving  how  small  a  quantity  of  ponderable  matter  under  cer- 
tain circumstances  will  occupy  great  space.  And  common  air 
is  by  no  means  the  lightest  known  substance,  which  as  power- 
fully resists  the  intrusion  of  other  bodies  where  it  exists.  Hy- 
drogen gas,  for  instance,,  of  the  same  space-occupying  force, 
weighs  only  a  fourteenth  part  as  much,  and  therefore  a  few 
drachms  of  it  confined  in  a  bag  or  bed  as  broad  as  the  founda- 
tion of  a  house,  would  support  a  house  or  a  cask  as  large  as  a 
house  filled  with  water  to  a  height  of  thirty  feet,  the  gas  itself 
being  then  eighty  thousand  times  lighter  than  its  bulk  of  gold; 
— and  if  the  pressure  on  it  were  diminished,,  it  would  readily 
expand  to  a  volume  a  thousand  times  as  great,  and  would  still 
be  exerting  a  considerable  outward  elasticity.  Again,  a  mix- 
ture of  oxygen  and  hydrogen  gases,  while  uniting  with  explo- 
sive force  to  form  water,  dilates  for  the  time,  even  under  the 
great  pressure  of  the  atmosphere,  to  a  bulk  about  twenty  times 
greater  than  the  gases  have  while  separate. 

The  mind,  pursuing  the  idea  of  such  expansion  or  occupancy 
of  space  by  a  small  quantity  of  matter,  and  reflecting  on  the 
wonderful  divisibility  of  matter  or  minuteness  of  the  ulti- 
mate atoms,  as  explained  in  Part  I.  of  this  work,  might  almost 
admit  as  a  possible  reality  Newton's  hypothetical  illustration  of 
that  divisibility,  viz.  that  even  one  ounce  of  substance  uniform- 
ly distributed  over  the  vast  space  in  which  our  solar  system 
exists,  might  leave  no  quarter  of  an  inch  without  its  particle. 
Now  a  fluid  in  any  degree  approaching  in  rarity  to  this,  al- 
though it  might  press,  resist,  communicate  motion,  and  have 
other  influences  in  common  with  more  ponderable  matter,  would 
have  neither  weight  nor  inertia  discoverable  by  means  at  pre- 
sent known  to  man.  While  we  are  contemplating,  then,  or 
modifying  the  agencies  of  what  causes  the  phenomena  of  heat 
and  cold,  of  light  and  darkness,  of  electricity  in  its  forms  of 
thunder  and  lightning,  of  galvanism,  or  of  magnetism,  in  a 


SUBSTANCE.  11 

word,  the  most  striking  phenomena  of  nature,  we  may  be  deal- 
ing with  matter  of  the  subtile  constitution  now  spoken  of. 
And  as  in  the  terrestrial  atmosphere  there  are  at  least  two  fluids 
present,  viz.  oxygen  and  nitrogen,  of  distinct  nature,  so  in  a 
more  subtile  ether  filling  all  space,  there  may  be  various  in- 
gredients. 

A  majority  of  philosophers  now  incline  to  the  opinion  here 
sketched,  that  there  is  at  least  one  such  subtile  fluid  or  ether 
occupying  completely  the  space  of  the  universe,  and  tend- 
ing to  uniform  diffusion  by  reason  of  a  strong  mutual  repulsion 
of  its  particles,  which  fluid  pervades  denser  material  substances 
somewhat  as  water  pervades  a  sponge  or  a  mass  of  sand,  being 
attracted  in  a  peculiar  way  by  each  substance,  and  which  fluid 
may  or  may  not  have  weight  and  inertia.  They  believe  far- 
ther that  the  phenomena  above  alluded  to,  and  which  human 
art  can  exhibit  with  highest  beauty,  or  with  awful  intensity, 
are  produced  by  the  motion  of  other  affections  of  that  fluid, 
as  the  sensation  of  sound  in  all  its  varieties  is  produced  in  the 
delicate  structure  of  the  ear  by  a  certain  motion  in  the  air,  or 
in  any  other  body,  having  communication  with  the  ear;  or  as 
the  sensation  of  jar  is  perceived  by  a  hand  held  to  one  end  of 
a  log  of  wood  when  a  blow  is  given  to  the  other  end.  Some 
philosophers  again  suppose  that  the  causes  of  the  phenomena 
are  material  particles  projected  through  space,  somewhat  as  sand 
might  be  scattered  by  an  explosion,  and  which  particles  are  pre- 
sent only  when  the  effects  are  apparent.  Some  combine  these 
two  hypotheses.  And  some  hold  all  the  phenomena  of  heat 
to  be  mere  motions  in  the  common  matter  of  the  bodies  in 
which  the  heat  exists. 

We  mention  these  hypotheses,  not  with  the  view  of  enter- 
ing upon  a  minute  examination  of  their  respective  merits,  or 
even  of  asserting  that  any  one  of  them  is  true,  but  merely  to 
make  the  reader  aware  of  the  directions  which  inquirers'  minds 
have  taken  in  pursuing  the  investigation.  To  understand  the 
subjects  as  far  as  men  yet  usefully  understand  them,  and  suffi- 
ciently for  a  vast  number  of  most  useful  purposes,  it  is  only 
necessary,  as  in  other  departments  of  science,  to  classify  im- 
portant phenomena,  so  that  their  nature  and  resemblances  may 


12 


IMPONDERABLE  SUBSTANCE. 


be  clearly  perceived.  When  in  treating  of  the  human  mind 
we  speak  of  its  retaining  an  idea,  or  being  depressed,  or  being 
heated  with  passion,  &c.,  we  speak  of  subjects  sufficiently  de- 
finite, although  we  may  have  no  hypothesis  as  to  the  intimate 
nature  of  the  phenomena: — and  in  the  same  manner  may  we 
speak  of  the  accumulation,  radiation,  or  other  affections  of  heat 
and  light.  We  know  nothing  of  the  cause,  even  of  gravity, 
the  grandest  influence  in  nature,  but  we  can  calculate  its  effects 
with  admirable  precision. 


[     13      ] 


PART  FOURTH. 


SECTION  I.— ON  HEAT. 


ANALYSIS    OF  THE  SECTION. 

Heat  (by  some  called  Caloric}  may  be  strikingly  referred  to  as  that 
which  causes  the  difference  between  winter  and  summer,  between 
tropical  gardens  and  polar  wastes.  Its  inferior  degrees  are  denoted 
by  the  term  COLD.     It  cannot  be  exhibited  apart,  nor  proved  to 
have  weight  or  inertia,  and  the  change  of  its  quantity  in  bodies 
is  most  conveniently  estimated  by  the  concomitant  change  of  their, 
bulk;  any  substance  so  circumstanced  as  to  allow  this  to  be  accu- 
rately measured  constituting  a  THERMOMETER. 
Heat  diffuses  itself  among  neighbouring  bodies  until  all  have  the 
same  temperature,  that  is,  until  all  similarly  affect  a  thermometer. 
It  spreads  partly  through  their  structure,  or  by  conduction,  as  it 
is  called,  tvith  a  slow  progress,  different  for  each  substance,  and 
in  fluids  modified  by  the  motion  of  their  particles;  and  it  spreads 
partly  also  by  being  shot  or  radiated  like  light  from  one  body  to 
another,  through  transparent  media  or  space,  with  readiness  af- 
fected by  the  material  and  state  of  the  giving  and  receiving  sur- 
faces. 

Heat,  by  entering  bodies,  expands  them,  and  through  a  range  which 
includes,  as  three  successive  stages,  the  forms  of  SOLID,  LIQUID, 
and  AIR  or  GAS;  becoming  thus  in  nature  the  grand  antagonist  and 
modifier  of  that  attraction  which  holds  corporeal  particles  together, 
and  which,  if  acting  alone,  would  reduce  the  whole  material  uni- 
verse to  one  solid  lifeless  mass.  Each  particular  substance,  accord- 
ing to  the  nature,  proximity,  fyc.  of  its  ultimate  particles,  takes  a 
certain  quantity  of  heat  (said  to  mark  its  capacity,)  to  produce  in 
it  a  given  change  of  temperature  or  calorific  tension;  undergoing 
expansion  then  in  a  degree  proper  to  itself,  and  changing  its  form 
to  liquid  and  air  at  points  of  temperature  proper  to  itself; — the  ex- 
pansion in  bodies  generally  increasing  more  rapidly  than  the  tem- 
perature, because  the  cohesion  of  their  particles  lessens  with  in- 


14  HEAT. 

crease  of  distance;  being  remarkably  greater  therefore  in  liquids 
than  in  solids,  and  in  airs  than  in  liquids;  and  the  rate  of  expan- 
sion, moreover,  being  much  quickened  as  the  bodies  approach  their 
points  of  changing  form  to  liquid  or  air,  to  produce  which  changes, 
a  large  quantity  of  heat  enters  them,  but  in  the  new  arrangement 
of  particles  and  increased  volume  of  the  mass,  it  becomes  hidden 
from.the  thermometer,  and  is  therefore  called  LATENT  HEAT.  For 
any  given  substance  the  changes  of  form  happen  so  constantly  at 
the  same  temperature,  that  they  mark  fixed  points  in  the  general 
scale  of  temperature,  and  enable  us  to  regulate  and  compare  ther- 
mometers.— Heat,  by  expanding  different  substances  unequally,  in- 
fluences much  their  chemical  combination. — Heat  influences  also 
the  functions  of  vegetable  and  animal  life. 

The  great  source  of  heat  is  the  sun;  but  electricity,  combustion,  and 
other  chemical  actions,  condensation,  friction,  and  the  actions  of 
life,  are  also  excitants.* 


"  Heat  may  be  strikingly  referred  to  as  that  which  causes 
the  difference  between  winter  and  summer,  between  the 
gardens  of  the  equator  and  polar  wastes."  (See  the  Ana- 
lysis, page  13.) 

In  the  winter  of  climates,  where  the  temperature  is  for  a 
time  below  the  freezing  point  of  water,  the  earth  with  its 
waters  is  bound  up  in  snow  and  ice,  the  trees  and  shrubs  are 
leafless,  appearing  every  where  like  withered  skeletons,  count- 
less multitudes  of  living  creatures,  owing  either  to  the  bitter 
cold  or  deficiency  of  food,  are  perishing  in  the  snows — na- 
ture seems  dying  or  dead;  but  what  a  change  when  spring  re- 
turns, that  is,  when  heat  returns!  The  earth  is  again  uncovered 
and  soft,  the  rivers  flow,  the  lakes  are  again  liquid  mirrors,  the 
warm  showers  come  to  foster  vegetation,  which  soon  covers  the 

*  It  is  to  be  remarked  here,  that  many  phenomena  in  which  heat  plays  an 
important  part,  have  been  already  described  in  preceding  chapters  of  this  work; 
— for  instance,  the  action  of  the  steam-engine,  the  phenomena  of  winds,  many 
facts  in  meteorology,  &c.  under  the  head  of  Pneumatics.  In  a  separate  trea- 
tise on  heat,  these  could  not  with  propriety  have  been  omitted;  but  in  a  com- 
prehensive system  of  science  like  the  present,  they  find  their  fit  place,  where, 
being  surrounded  by  subjects  resembling  them  in  more  intricate  particulars, 
they  can  be  more  concisely  and  clearly  explained. 


CAUSE'OF  SEASONS  AND  CLIMATES.  15 

ground  with  beauty  and  plenty.     Man,  lately  inactive,  is  re- 
called to  many  duties;  his  water-wheels  are  every  where  at 
work,  his  boats  are  again  on  the  canals  and  streams,  his  busy 
fleets  of  industry  are  along  the  shores: — winged  life  in  new 
multitudes  fills  the  sky,  finny  life  similarly  fills  the  waters,  and 
every  spot  of  earth  teems  with  vitality  and  joy.     Many  persons 
regard  these  changes  of  season  as  if  they  came  like  the  succes- 
sive   positions  of  a  turning  wheel,,  of  which  one  necessarily 
brings  the-  next;  not  adverting  that  it  rs  the  single  circumstance 
of  change  of  temperature,  which  does  all.     But  rf  the  colds  of 
winter  arrive  too  early,  they  unfailingly  produce  the  wintry 
scene,  and  if  warmth  come  before  its  time  in  spring,  it  expands 
the  bud  and  the  blossom,  which  a  return  of  frost  will  surely 
destroy.     A  seed  sown  in  an  ice-house  never  awakens  to  life. 
Again,  as  regards  climates,  the  earthy  matters  forming  the 
exterior  of  our  globe,  and  therefore  entering  into  the  composi- 
tion of  soils,  are  not  different  for  different  latitudes, — at  the 
equator,  for  instance,  and  near  the  poles.   That  the  aspect  of  na- 
ture then  in  the  two  situations  exhibits  a  contrast  more  striking 
still  than  between  summer  and  winter,  is  merely  to  an  inequa- 
lity of  temperature,  which  is  permanent.     Were  it  not  for  this, 
in  both  situations  the  same  vegetables  might  grow,  and  the  same 
animals  might  find  their  befitting  support.     But  now,  in  the 
one,  namely,  where  heat  abounds,  we  seethe  magnificent  scene 
of  tropical  fertility:  the  earth  covered  with  luxuriant  vegeta- 
tion in  endless  lovely  variety,  and  even  the  hard  rocks  festooned 
with  green,  perhaps  with   the  vine,  rich  in  its  purple  clusters. 
In  the  midst  of  this  scene,  animal  existence  is  equally  abundant, 
and  many  of  the  species  are  of  surpassing  beauty — the  plumage 
of  the  birds  is  as  brilliant  as  the  gayest  flowers.     The  warm  air 
is  perfume  from  the  spice-beds,  the  sky  and  clouds  are  often 
dyed  in  tints  as  bright  as  freshest  rainbow,  and  happy  human 
inhabitants  call  the  scene  a  paradise.     Again,  where  heat  is  ab- 
sent, we  have  the  dreary  spectacle  of  polar  barrenness,  namely, 
bare  rock  or   mountain,  instead  of  fertile  field;  water  every 
where  hardened  to  solidity,  no  rain,  nor  cloud,  nor  dew,  few 
motions  but  drifting  snow;  vegetable   life  scarcely  existing, 
and  then  only  in  sheltered  places  turned  to  the  sun — and  in- 


16  HEAT. 

stead  of  the  palms  and  other  trees  of  India,  whose  single  leaf  is 
almost  broad  enough  to  cover  a  hut,  there  are  bushes  and 
trees,  as  the  furze  and  fir,  having  what  may  be  called  hairs  or 
bristles,  in  the  room  of  leaves.  In  the  winter  time,  during 
which  the  sun  is  not  seen  for  nearly  six  months,  new  horrors 
are  added,  viz.  the  darkness  and  dreadful  silence,  the  cold  be- 
numbing all  life,  and  even  freezing  mercury — a  scene  into  which 
man  may  penetrate  from  happier  climes,  but  where  he  can  only 
leave  his  protecting  ship  and  fires  for  short  periods,  as  he  might 
issue  from  a  diving-bell  at  the  bottom  of  the  ocean.  That  in 
these  now  desolate  regions,  heat  only  is  wanted  to  make  them 
like  the  most  favoured  countries  of  the  earth,  is  proved  by  the 
recent  discoveries  under  ground  of  the  remnant  of  animals  and 
vegetables  formerly  inhabiting  them,  which  now  can  live  only 
near  the  equator.  While  winter  then,  or  the  temporary  ab- 
sence of  heat,  may  be  called  the  sleep  of  nature,  the  more  per- 
manent torpor  about  the  poles  appears  like  its  death;  and  when 
we  farther  reflect,  that  heat  is  the  great  agent  in  numberless  im- 
portant processes  of  chemistry  and  domestic  economy,  and  is 
the  actuating  principle  of  the  mighty  steam  engine  which  now 
performs  half  the  work  of  society,  how  truly  may  heat,  the  sub- 
ject of  our  present  chapter,  be  considered  as  the  life  or  soul  of 
the  universe! 

"  Heat  cannot  be  exhibited  in  a  separate  state,  nor  proved 
to  have  weight  or  inertia."  (Read  the  Analysis,  page  13.) 

Although  heat  is  known  to  be  abundant  in  the  sun-beam,  and 
to  radiate  around  from  a  blazing  fire,  we  cannot  otherwise  ar- 
rest or  detect  it  in  its  progress  than  by  allowing  it  to  enter,  and 
remain  in  some  ponderable  substance.  We  know  hot  iron  or 
hot  water  or  hot  air,  but  nature  nowhere  presents  to  us,  nor 
has  art  succeeded  in  showing  us  heat  alone. 

If  we  balance  a  quantity  of  ice  in  a  delicate  weigh-beam,  and 
then  leave  it  to  melt,  the  equilibrium  will  not  be  in  the  slight- 
est degree  disturbed.  Or  if  we  substitute  for  the  ice,  boiling 
water  or  red-hot  iron,  and  leave  this  to  cool,  there  will  be 
no  difference  in  the  result.  If  we  place  a  pound  of  mercury  in 
one  scale  of  the  weigh-bean  and  a  pound  of  water  in  the  other, 


HAS  XO  WEIGHT  OR  INERTIA.  17 

and  then  either  heat  or  cool  both  through  the  same  number  of 
thermometrlc  degrees,  although  about  thirty  times  moreheat  (as 
will  be  explained  below  (enters  or  leaves  the  bulky  water  than 
the  dense  mercury,  they  will  still  remain  equivalent  weights. 

Again,  a  sun-beam,  with  its  intense  light  and  heat,  after  being 
concentrated  by  a  powerful  lens  or  mirror,  may  be  made  to  fall 
upon  the  scale  of  a  most  delicate  balance,  but  will  produce  no 
depressing  effect  on  the  scale,  as  would  follow  if  what  consti- 
tutes the  beam  had  the  least  forward  motal  inertia  or  momen- 
tum. 

Such  are  the  facts  which  have  led  certain  inquirers  to  deny 
the  material  or  separate  existence  of  heat,  and  to  hold  that  it  is 
merely  motion  of  one  kind  among  the  material  particles  of  bo- 
dies generally,  as  sound  is  motion  of  another  kind  among  the 
same  particles.  The  following  facts  they  consider  to  have  the 
same  bearing  in  the  argument.  Heat  can  be  produced  without 
limit  by  friction,  as — when  savages  light  their  fires  by  rubbing 
together  two  pieces  of  wood — when  Count  Rum  ford  made  great 
quantities  of  water  boil,  by  causing  a  blunt  borer  to  rub  against 
a  mass  of  metal  immersed  in  the  water — when  Sir  Humphrey 
Davy  quickly  melted  pieces  of  ice  by  rubbing  them  against 
each  other  in  a  room  cooled  below  the  freezing  point,  &c.  In- 
tense heat  is  produced  by  the  explosion  of  gunpowder  or  other 
fulminating  mixture,  yet  it  cannot  be  conceived  to  have  existed 
in  the  small  bulk  of  the  powder  before  the  explosion.  Other 
inquirers,  on  the  contrary,  have  deemed  to  be  proofs  of  the 
separate  materiality  of  heat  such  facts  as  now  follow: — that  it  is 
radiated  through  the  most  perfect  vacuum  which  we  can  pro- 
duce, and  even  more  readily  than  through  air;— that  it  radiates 
in  the  same  place  in  all  directions,  without  impediment  from 
the  crossing  rays; — that  it  becomes  instantly  sensible  on  the 
condensation  of  any  material  mass,  as  if  then  squeezed  out  from 
the  mass;  as  when,  by  compressing  air  suddenly,  we  inflame  a 
match  immersed  in  it;  or  when,  on  reducing  the  bulk  of  iron 
by  hammering,  we  render  it  very  hot,  the  warming  being 
greater  at  the  first  blow  (which  most  changes  the  bulk)  than 
afterwards, — that  when,  on  mixing  bodies  which  combine  so 
intimately  as  to  occupy  less  space  than  when  separate,  there  is 

3 


16  H£AT- 

a  disengagement  of  heat  proportioned  to  the  diminution  of  vo- 
lume:— that  the  laws  of  the  spreading  of  heat  in  bodies  do  not 
resemble  those  of  the  spreading  of  sound,  or  of  any  other  mo- 
tion known  to  us: — and  that,  as  to  the  great  and  sudden  extri- 
cation of  heat  by  friction  or  explosion,  it  may  be  as  truly  a  rush 
of  the  fluid  to  the  part,  as  in  the  case  of  an  electrical  accumula- 
tion or  discharge.  These  facts,  moreover,  they  think,  square 
well  with  their  assumption  that  the  phenomena  of  heat  are  pro- 
duced by  an  exceedingly  subtile  fluid,  or  ether,  pervading  the 
whole  universe,  and  softening  or  melting  or  gasifying  bodies, 
according  to  the  quantity  present  in  each;  its  own  parts  being 
strongly  repulsive  of  each  other,  and  seeking,  therefore,  widest 
and  most  equable  diffusion. 

i:  The,  change  of  its  quantity  in  bodies  is  most  conveniently 
estimated  by  the.  concomitant  change  of  their  bulk,  any 
substance  so  circumstanced,  as  to  allow  this  to  be  accu- 
rately measured,  constituting  a  thermometer."  (Read 
the  Analysis,  page  13.) 

If  we  heat  a  wire,  it  is  lengthened;  if  we  heat  water  in  a  full 
vessel,  a  part  runs  over;  if  we  heat  air  in  a  bladder,  the  bladder 
is  distended:  in  a  word,  if  we  heat  any  substance,  its  volume 
increases  in  some  proportion  to  the  increase  of  temperature,— 
and  we  may  measure  the  increase  of  volume.  The  reasons 
why,  in  such  investigations,  a  contrivance  in  which  the  expan- 
sion of  mercury  may  be  observed,  viz.  the  mercurial  thermo- 
meter, is  commonly  preferred  to  others,  can  only  be  fully  un- 
derstood by  the  mind  which  has  considered  the  whole  subject 
of  heat;  and  we  touch  upon  the  matter  here,  only  for  the  pur- 
pose of  stating  that  a  mercurial  thermometer  is  a  small  bulb, 
or  bottle  of  glass  filled  with  mercury,  and  having  a  long  very 
narrow  stalk  or  neck,  in  which  the  mercury  rises  when  expand- 
ed by  heat,  or  falls  when  heat  is  withdrawn;  the  stalk  between 
the  points  at  which  the  mercury  stands  in  freezing  and  in  boil- 
ing water,  being  divided  into  an  arbitrary  number  of  degrees, 
which  division  appearing  on  a  scale  applied  to  the  stalk,  is  con- 
tinued similarly  above  and  below  these  points. 


SPKEABIXG    BY   CO^DUCTIOX.  19 

€€  Heat  diffuses  itself  among  neighbouring  bodies  until  all 
have  acquired  the  same  temperature;  that  is  to  say,  until 
all  will  similarly  affect  a  thermometer."  (See  the  Analy- 
sis, page  13.) 

An  iron  bolt  thrust  in  among  burning  coals  soon  becomes  red 
hot  like  them.  If  it  be  the  heater  of  a  tea-urn,  it  will,  when 
afterwards  placed  amidst  the  water,  part  with  its  lately  acquired 
heat  to  the  water,  until  both  are  of  the  same  temperature.  Boil- 
ing water,  again,  soon  imparts  heat  to  an  egg  placed  in  it,  and  a 
feverish  head  yields  its  heat  to  a  bladder  of  cold  water  or  ice. 
A  hundred  objects  enclosed  in  the  same  apartment,  if  tested, 
after  a  time,  by  the  thermometer,  will  all  indicate  the  same 
temperature. 

'*  The  inferior  degrees  of  heat  are  denoted  by  the  term  COLD." 
When  the  hand  touches  a  body  of  higher  temperature  than  it- 
self, it  receives  heat  according  to  the  law  now  explained,  and  it 
experiences  a  peculiar  sensation;  when  it  touches  a  body  of 
lower  temperature  than  itself,  it  gives  out  heat  for  a  like  reason, 
and  experiences  another  and  very  different  sensation.  The  two 
are  called  the  sensations  of  heat  and  of  cold.  Now  heat  and 
cold,  considered  as  existing  in  the  bodies  themselves,  although 
thus  appearing  opposites,  are  really  degrees  of  the  same  object, 
temperature,  contrasted  by  name  for  convenience  sake,  in  re- 
ference to  the  particular  temperature  of  the  individuals  speaking 
of  them — just  as  any  two  nearest  mile-stones  on  a  road,  although 
merely  marking  degrees  of  the  same  object,  distance,  might 
receive  from  persons  living  between  them  the  opposite  names  of 
east  and  west,  or  of  north  and  south.  It  is  to  be  remarked, 
moreover,  that  the  sensation  of  heat  is  producible  also  by  a 
body  colder  than  the  hand,  provided  it  be  less  cold  than  a  body 
touched  immediately  before,  or  than  the  usual  temperature;  and 
the  sensation  of  cold  is  producible  under  the  opposite  circum- 
stances of  touching  a  comparatively  warm  body,  but  which  is 
less  warm  than  something  touched  just  before.  This  explains 
the  remarkable  fact  that  the  same  body  may  appear  at  the  same 
time,  and  to  the  same  person,  both  hot  and  cold.  If  a  person 
transfer  one  hand  to  common  spring-water  from  touching  ice, 


that  hand  will  deem  the  water  very  warm;  while  the  other 
hand,  transferred  to  it  from  a  warm  bath,  would  deem  it  very 
cold.  For  a  like  reason,  a  person  from  India,  arriving  in  Eng- 
land in  the  spring,  deems  the  air  cold,  while  the  inhabitants  of 
the  country  are  diminishing  their  clothing,  because  the  heat  to 
them  is  becoming  oppressive.  Such  facts  show  how  necessary 
it  was  for  men  to  discover  more  correct  thermometers  than  their 
bodily  sensations. 

"  Spreading  partly  through  their  structure,  or  by  conduc- 
tion, as  it  is  called,  with  a  progress  proper  to  each  sub- 
stance"    (Read  the  Analysis,  page  13.) 
If  one  end  of  a  rod  of  iron  be  held  in  the  fire,  a  hand  grasping 
the  other  end  soon  feels  the  heat  coming  through  it.   Through  a 
similar  rod  of  glass  the  transmission  is  much  slower,  and  through 
one  of  wood  it  is  slower  still.     The  hand  would  be  burned  by 
the  iron,  before  it  felt  warmth  in  the  wood,  although  the  inner 
end  were  blazing. 

On  the  fact  that  different  substances  are  permeable  to  heat,  or 
have  the  property  of  conducting  it,  in  different  degrees,  depend 
many  interesting  phenomena  in  nature  and  in  the  arts:  hence 
it  was  important  to  ascertain  the  degrees  exactly,  and  to  classify 
the  substances.  Various  methods  for  this  purpose  have  been 
adopted.  For  solids — similar  rods  of  the  different,  substances, 
after  being  thinly  coated  with  wax,  have  been  placed  with  their 
inferior  extremities  in  hot  oil,  and  then  the  comparative  dis- 
tances to  which,  in  a  given  time,  the  wax  was  melted,  furnished 
one  set  of  indications  of  the  comparative  conducting  powers: — 
or,  equal  lengths  of  the  different  bare  rods  being  left  above  the 
oil,  and  a  small  quantity  of  explosive  powder  being  placed  on 
the  top  of  each,  the  comparative  intervals  of  time  elapsing  be- 
fore the  explosions  gave  another  kind  of  measure: — or,  equal 
balls  of  different  substances,  with  a  central  cavity  in  each  to  re- 
ceive a  thermometer,  being  heated  to  the  same  degree  and  then 
suspended  in  the  air  to  cool,  until  the  thermometer  fell  to  a 
given  point,  gave  still  another  list.  A  modification  of  the  last 
method  was  adopted  by  Count  Rumford  to  ascertain  the  relative 
degrees  in  which  furs,  feathers,  and  other  materials  used  for 


•SPREADING-   BY   COSDTJCTIO^. 


clothing,  conduct  heat,  or,  which  is  the  same  thing,  resist  its 
passage.     He  covered  tne  ball  and  stem  of  a  thermometer  with 
a  certain  thickness  of  the  substance  to  be  tried,  by  placing  the 
thermometer  in  a  larger  bulb  and  stem  of  glass,  and  then  filling 
the  interval  between  them  with  the  substance;  and,  after  heat- 
ing this  apparatus  to  a  certain  degree,  by  dipping  it  in  liquid  of 
the  desired  temperature,  he  surrounded  it  by  ice,  and  marked 
the  comparative  times  required  to  cool  the  thermometer  a  cer- 
tain number  of  degrees.     The  figures  following  the  names  of 
some  of  the  substances  in  the  subjoined  list,  mark  the  number 
of  seconds  required  respectively  for  cooling  it  60°. 

These  experiments  have  shown  as  a  general  rule,  that  den- 
sity in  a  body  favours  the  passage  of  heat  through  it.     The 
best  conductors  are  the  metals,  and  then  follow  in  succession 
diamond,  glass,  stones,  earths,  woods,  &c.,  as  here  noted: 
Metals  —  silver,  copper,  gold,  iron,  lead. 
Diamond. 
Glass. 

Hard  stones. 
Porous  earths. 
Woods. 

Fats  or  thick  oils. 
Snow. 
Air    .....          ...     57tf 

Sewing  silk       ------      917 

Wood  ashes      ----..     937 

Charcoal       -.-....     937 

Fine  lint      .......   1,032 

Cotton     ........  1,046 

Lamp  black      ......  1,117 

Wool       ........  i,H8 

Raw  silk      .......  1,284 

Beavers'  fur     ......  1,296 

Eider  down      ......  1,305 

Hares'  fur    .......  1,315 

Air  appears  near  the  middle  of  the  preceding  list,  but  if  its 
particles  are  not  allowed  to  move  about  among  themselves,  so 
as  to  carry  heat  from  one  part  to  another,  it  conducts  (in  the 


,'  3  HEAT. 

manner  of  solids)  so  slowly  that  Count  Rumford  doubted  whe- 
ther it  conducted  at  all.  It  is  probably  the  worst  conductor 
known,  that  is,  the  substance  which  when  at  rest  impedes  the 
passage  of  heat  the  most.  To  this  fact  seems  to  be  owing  in 
a  considerable  degree  the  remarkable  non-conducting  quality 
of  porous  or  spongy  substances,  as  feathers,  loose  filamentous 
matter,  powders,  &c.,  which  have  much  air  in  their  structure, 
often  adherent  with  a  force  of  attraction  which  immersion  in 
water,  or  even  being  placed  in  the  vacuum  of  an  air-pump,  is 
insufficient  to  overcome. 

While  contemplating  the  facts  recorded  in  the  above  table, 
one  cannot  but  reflect  how  admirably  adapted  to  their  purposes 
the  substances  are  whieh  nature  has  provided  as  clothing  for 
the  inferior  animals; — and  which  man  afterwards  accommodates 
with  such  curious  art  to  his  peculiar  wants.  Animals  required 
to  be  protected  against  the  chills  of  night  and  the  biting  blasts 
of  winter;  and  some  of  them  which  dwell  among  eternal  ice, 
could  not  have  lined  at  all,  but  for  a  garment  which  might  shut 
up  within  it  nearly  all  the  heat  which  their  vital  functions  pro- 
duced. Now  any  covering  of  a  metallic  or  earthy  or  woody 
nature,  would  have  been  far  from  sufficing;  but  out  of  a  won- 
drous chemical  union  of  carbon  with  the  soft  ingredients  of 
the  atmosphere,  those  beautiful  textures  are  produced  called  fur 
and  feather,  so  greatly  adorning  while  they  completely  protect 
the  wearers: — textures,  moreover,  which  grow  from  the  bodies 
of  the  animals,  in  the  exact  quantity  that  suits  the  climate  and 
season,  and  which  are  reproduced  when  by  any  accident  they  are 
partially  destroyed.  In  warm  climates  the  hairy  coat  of  quad- 
rupeds is  comparatively  short  and  thin;  as  in  the  elephant, 
the  monkey,  the  tropical  sheep,  &c.  It  is  seen  to  thicken 
With  increasing  latitude,  furnishing  the  soft  and  abundant  fleeces 
of  the  temperate  zones;  and  towards  the  poles  it  is  externally 
shaggy  and  coarse,  as  in  the  arctic  bear.  In  amphibious  ani- 
mals, which  have  to  resist  the  cold  of  water  as  well  as  of  air, 
the  fur  grows  particularly  defensive,  as  in  the  otter  and  bea- 
ver. Birds,  from  having  very  warm  blood,  required  plenteous 
clothing,  but  required  also  to  have  a  smooth  surface,  that  they 
might  pass  easily  through  the  air: — both  objects  are  securectby 


BY  CONDUCTION. 

the  beautiful  structure  of  feathers,  so  beautiful  and  wonderful 
that  writers  on  natural  theology  have  often  particularized  it  as 
one  of  the  most  striking  exemplifications  of  creative  wisdom. 
Feathers,  like  fur,  appear  in  kind  and  quantity  suited  to  parti- 
cular climates  and  seasons.  The  birds  of  cold  regions  have 
covering  almost  as  bulky  as  their  bodies,  and  if  it  be  warm  in 
those  of  them  which  live  only  in  air,  in  the  water-fowl  it  is 
warmer  still.  These  last  have  the  interstices  of  the  ordinary 
plumage  filled  up  by  the  still  more  delicate  structure  called 
down,  particularly  on  the  breast,  which  in  swimming  first 
meets  and  divides  the  cold  wave.  There  are  animals  with 
warm  blood  which  yet  live  very  constantly  immersed  in  water, 
as  the  whale,  seal,  walrus,  &c.  Now  neither  hair  nor  feathers, 
however  oiled,  would  have  been  a  fit  covering  for  them;  but 
kind  nature  has  prepared  an  equal  protection  in  the  vast  mass 
of  fat  or  thick  oil  which  surrounds  their  bodies — substances 
which  are  scarcely  less  useful  to  man  than  the  furs  and  fea- 
thers of  land  animals. 

While  speaking  of  clothing,  we  may  remark,  that  the  bark 
of  trees  is  also  a  structure  very  slowly  permeable  to  heat, 
and  securing  therefore  the  temperature  necessary  to  vegetable 
life. 

And  while  we  admire  what  nature  has  thus  done  for  animals 
and  vegetables,  let  us  not  overlook  her  scarcely  less  remarkable 
provision  of  ice  and  snow,  as  winter  clothing  for  the  lakes  and 
rivers,  for  our  fields  and  gardens.  Ice,  as  a  protection  to  wa- 
ter and  its  inhabitants,  was  considered  in  vol.  i.  in  the  explana- 
tion of  why,  although  solid,  it  swims  on  water.  We  have  now 
to  remark  that  snow,  which  becomes  as  a  pure  white  fleece  to 
the  earth,  is  a  structure  which  resists  the  passage  of  heat  near- 
ly as  much  as  feathers.  It,  of  course,  can  defend  only  from 
colds  below  32°  or  the  freezing  point;  but  it  does  so  most  ef- 
fectually, preserving  the  roots  and  seeds  and  tender  plants 
during  the  severity  of  winter.  When  the  green  blade  of  wheat 
and  the  beautiful  snow-drop  flower  appear  in  spring  rising 
through  the  melting  snow,  they  have  recently  owed  an  import- 
ant shelter  to  their  wintry  mantle.  Under  deep  snow,  while 
the  thermometer  in  the  air  may  be  far  below  zero,  the  tempera- 


oi-  HEAT. 

ture  of  the  ground  rarely  remains  below  the  freezing  point 
Now  this  temperature,  to  persons  some  time  accustomed  to  it, 
is  mild  and  even  agreeable.  It  is  much  higher  than  what  of- 
ten prevails  for  long  periods  in  the  atmosphere  of  the  centre 
and  north  of  Europe.  The  Laplander,  who  during  his  long 
winter  lives  under  ground,  is  glad  to  have  additionally  over- 
head a  thick  covering  of  snow.  Among  the  hills  of  the  west 
and  north  of  Britain,  during  the  storms  of  winter,  a  house  or 
covering  of  snow  frequently  preserves  the  lives  of  travellers, 
and  even  of  whole  flocks  of  sheep,  when  the  keen  north  wind 
catching  them  unprotected,  would  soon  stretch  them  lifeless 
along  the  earth. 

It  is  because  earth  conducts  heat  slowly,  that  the  most  intense 
frosts  penetrate  but  a  few  inches  into  it,  and  that  the  tempera- 
ture of  the  ground  a  few  feet  below  its  surface  is  nearly  the 
same  all  the  world  over.  In  many  mines,  even  although  open 
to  the  air,  the  thermometer  does  not  vary  one  degree  in  a 
twelvemonth.  Thus  also  water  in  pipes  two  or  three  feet  un- 
der ground  does  not  freeze,  although  it  may  be  frozen  in  all  the 
smaller  branches  exposed  above.  Hence,  again,  springs  never 
freeze,  and  therefore  become  remarkable  features  in  a  snow  co- 
vered country.  The  living  water  is  seen  issuing  from  the  bowels 
of  the  earth,  and  running  often  a  considerable  way  through 
fringes  of  green,  before  the  gripe  of  the  frost  arrests  it;  while 
around  it,  as  is  well  known  to  the  sportsman,  the  snipes  and  wild 
duck  and  other  birds  are  wont  to  congregate.  A  spring  in  a  fro- 
zen pond  or  lake  may  cause  the  ice  to  be  so  thin  over  the  part 
where  it  issues,  that  a  skaiter  arriving  there  will  break  through 
and  be  destroyed.  The  same  spring  water  which  appears  warm 
in  winter,  is  deemed  cold  in  summer,  because,  although  always 
of  the  same  heat,  it  is  in  summer  surrounded  by  warmer  atmos- 
phere and  objects.  In  proportion  as  buildings  are  massive, 
they  acquire  more  of  those  qualities,  which  have  now  been  no- 
ticed of  our  mother  earth.  Many  of  the  gothic  halls  and  ca- 
thedrals are  cool  in  summer  and  warm  in  winter — as  are  also  old 
fashioned  houses  or  castles  with  thick  walls  and  deep  cellars. 
Natural  caves  in  the  mountains  or  sea-shores  furnish  other  ex- 
amples of  a  similar  kind. 


SPItEADINtt  BY  CONDUCTION.  25 

When  in  the  arts  it  is  desired  to  prevent  the  passage  of  heat 
out  of  or  into  any  body  or  situation,  a  screen  or  covering  of  a 
slow  conducting  substance  is  employed.  Thus,  to  prevent  the 
heat  of  a  smelting  or  other  furnace  from  being  wasted,  it  is  lined 
with  fire  bricks,  or  is  covered  with  clay  and  sand,  or  sometimes 
with  powdered  charcoal.  A  furnace  so  guarded  may  be  touched 
by  the  hand,  even  while  containing  within  it  melted  gold.  To 
prevent  the  freezing  of  water  in  pipes  during  the  winter,  by 
which  occurrence  the  pipes  would  be  burst,  it  is  common  to 
cover  them  with  straw  ropes,  or  coarse  flannel,  or  to  enclose 
them  in  a  larger  outer  pipe,  with  dry  charcoal,  or  saw  dust,  or 
chaff,  filling  up  the  interval  between.  If  a  pipe,  on  the  con- 
trary, be  for  the  conveyance  of  steam  or  other  warm  fluid,  the 
heat  is  retained,  and  therefore  saved  by  the  very  same  means. 
Ice  houses  are  generally  made  with  double  walls,  between  which 
dry  straw  placed,  or  saw  dust,  or  air,  prevents  the  passage  of  heat. 
Pails  for  carrying  ice  in  summer,  or  intended  to  serve  as  wine 
coolers,  are  made  on  the  same  principle — viz.  double  vessels, 
with  air  or  charcoal,  filling  the  interval  between  them.  A  flan- 
nel covering  keeps  a  man  warm  in  winter — it  is  also  the  best 
means  of  keeping  ice  from  melting  in  summer.  Urns  for  hot 
water,  tea  pots,  coffee  pots,  &c.  are  made  with  wooden  or  ivory 
handles,  because  if  metal  were  used,  it  would  conduct  the  heat 
so  readily  that  the  hand  could  not  bear  to  touch  them. 

It  is  because  glass  and  earthenware  are  brittle,  and  do  not  al- 
low ready  passage  to  heat,  that  vessels  made  of  them  are  so  fre- 
quently broken  by  sudden  change  of  temperature.  On  pouring 
boiling  water  into  such  a  vessel,  the  internal  part  is  much  heated 
and  expanded  (as  will  be  explained  more  fully  in  a  subsequent 
page)  before  the  external  part  has  felt  the  influence,  and  this  IB 
hence  riven  or  cracked  by  its  connexion  with  the  internal.  A 
chimney  mirror  is  often  broken  by  a  lamp  or  candle  placed  on 
the  marble  shelf  too  near  it.  The  glass  cylinder  of  an  electri- 
cal machine  will  sometimes  be  broken  by  placing  it  near  the  fire, 
so  that  one  side  is  heated  while  the  other  side  receives  a  cold 
current  of  air  approaching  the  fire  from  a  door  or  window.  A 
red  hot  rod  of  iron  drawn  along  a  pane  of  glass  will  divide  it 
almost  like  a  diamond  knife.  Even  cast  iron,  as  backs  of  grates, 

4 


yti  -HEAT. 

iron  pots,  £c.,  although  conducting  readily,  is  often,  owing  to 
its  brittlcncss,  cracked  by  unequal  heating  or  cooling,  as  from 
pouring  water  on  it  when  hot.  Pouring  cold  water  into  a  heated 
glass  will  produce  a  similar  effect.  Hence  glass  vessels  intended 
to  be  exposed  to  strong  heats  and  sudden  changes,  as  retorts  for 
distillation,  flasks  for  boiling  liquids,  &c.,  are  made  very  thin, 
that  the  heat  may  pervade  them  almost  instantly  and  with  im- 
punity. 

There  is  a  toy  called  a  Prince  Rupert's  Drop,  which  well 
illustrates  our  present  subject.  It  is  a  lump  of  glass  let  fall 
while  fused  into  water,  and  thereby  suddenly  cooled  and  solidi- 
fied on  the  outside  before  the  internal  part  is  changed;  then  as 
this  at  last  hardens  and  would  contract,  it  is  kept  extended  by 
the  arch  of  external  crust,  to  which  it  coheres.  Now  if  a  por- 
tion of  the  neck  of  the  lump  be  broken  off,  or  if  other  violence 
be  done,  which  jars  its  substance,  the  cohesion  is  destroyed,  and 
the  whole  crumbles  to  dust  with  a  kind  of  explosion.  Any 
glass  cooled  suddenly  when  first  made,  remains  very  brittle,  for 
the  reason  now  stated.  -  What  is  called  the  Bologna  jar  is  a 
very  thick  small  bottle,  thus  prepared,  which  bursts  by  a  grain 
of  sand  falling  into  it.  The  process  of  annealing,  to  render 
glass  ware  more  tough  and  durable,  is  merely  the  allowing  it 
to  cool  very  slowly  by  placing  it  in  an  oven,  where  the  tempe- 
rature is  caused  to  fall  gradually.  The  tempering  of  metals  by 
sudden  cooling  seems  to  be  a  process  having  some  relation  to 
that  of  rendering  glass  hard  and  brittle. 

It  is  the  difference  of  conducting  power  in  bodies  which  is 
the  cause  of  a  very  common  error  made  by  persons  in  esti- 
mating the  temperature  of  bodies  by  the  touch.  In  a  room 
without  a  fire  all  the  articles  of  furniture  soon  acquire  the  same 
temperature;  but  if  in  winter,  a  person  with  bare  feet  were  to  step 
from  the  carpet  to  the  wooden  floor,  from  this  to  the  hearth- 
stone, and  from  the  stone  to  the  steel  fender,  his  sensation  would 
deem  each  of  these  in  succession  colder  than  the  preceding. 
Now  the  truth  being  that  all  had  the  same  temperature,  only  a 
temperature  inferior  to  that  of  the  living  body,the  best  conductor, 
when  in  contact  with  the  body,  would  carry  off  heat  the  fastest, 
and  would  therefore  be  deemed  the  coldest*  Were  a  similar 


.SPREADING  BY  CONBUOTIOX.  27 

experiment  made  in  a  hot  house,  or  in  India,  while  the  temper- 
ature of  every  thing  around  were  98°,  viz.  that  of  the  living 
body,  then  not  the  slightest  difference  would  be  felt  in  any  of 
the  substances:  or  lastly,  were  the  experiment  made  in  a  room 
where  by  any  means  the  general  temperature  were  raised  con- 
siderably above  blood  heat;  then  the  carpet  would  be  deemed 
considerably  the  coolest  instead  of  the  warmest,  and  the  other 
things  would  appear  hotter  in  the  same  order  in  which  they  ap- 
peared colder  in  the  winter  room.  Were  a  bunch  of  wool  and 
a  piece  of  iron  exposed  to  the  severest  cold  of  Siberia,  or  of  an 
artificial  frigorific  mixture,  a  man  might  touch  the  first  with 
impunity  (it  would  merely  be  felt  as  rather  cold;)  but  if  he 
grasped  the  second,  his  hand  would  be  frost  bitten  and  possibly 
destroyed:  were  the  two  substances  on  the  contrary,  transferred 
to  an  oven,  and  heated  as  far  as  the  wool  would  bear,  he  might 
again  touch  the  wool  with  impunity  (it  would  then  be  felt  as  a 
little  hot,)  but  the  iron  would  burn  his  flesh.  The  author  has 
entered  a  room  where  there  was  no  fire,  but  where  the  temper- 
ature from  hot  air  admitted  was  sufficiently  high  to  boil  the 
fish,  &c.  of  which  he  afterwards  partook  at  dinner;  and  he 
breathed  the  air  with  very  little  uneasiness.  He  could  bear  to 
touch  woollen  cloth  in  this  room,  but  no  body  more  solid. 

The  foregoing  considerations  make  manifest  the  error  of  sup- 
posing that  there  is  a  positive  warmth  in  the  materials  of  cloth- 
ing. The  thick  cloak  which  guards  a  Spaniard  against  the  cold 
of  winter,  is  also  in  summer  used  by  him  as  protection  against 
the  direct  rays  of  the  sun: — and  while  in  England  flannel  is  our 
warmest  article  of  dress,  yet  we  cannot  more  effectually  pre- 
serve ice  than  by  wrapping  the  vessel  containing  it  in  many 
folds  of  softest  flannel. 

In  every  case  where  a  substance  of  different  temperature 
from  the  living  body  touches  it,  a  thin  surface  of  the  substance 
immediately  shares  the  heat  of  the  bodily  part  touched — the 
hand  generally;  and  while  in  a  good  conductor,  the  heat  so  re- 
ceived quickly  passes  inwards,  or  away  from  the  surface,  leaving 
this  in  a  state  to  absorb  more,  in  the  tardy  conductor  the  heat 
first  received  tarries  at  the  surface,  which  consequently  soon  ac- 
quires nearly  the  same  temperature  as  the  hand,  and  therefore, 
cold  the  interior  of  the  substance  may  be,  it  does  not 


HEAT. 

cause  the  sensation  of  cold.  The  hand  on  a  good  conductor  has 
to  warm  it  deeply,  a  slow  conductor  it  warms  only  superficially. 
The  following  cases  farther  illustrate  the  same  principle.  If  the 
ends  of  an  iron  poker,  and  of  a  piece  of  wood  of  the  same  size, 
be  wrapped  in  paper  and  then  thrust  into  a  fire,  the  paper  on  the 
wood  will  begin  to  burn  immediately,  while  that  on  the  metal 
will  long  resist: — or  if  pieces  of  paper  be  laid  on  a  wooden 
plank  and  on  a  plate  of  steel,  and  then  a  burning  coal  be  placed 
on  each,  the  paper  on  the  wood  will  begin  to  burn  long  before 
that  on  the  plate.  The  explanation  is,  that  the  paper  in  contact 
with  the  good  conductor  loses  to  this  so  rapidly  the  heat  received 
from  the  coal,  that  it  remains  at  too  low  a  temperature  to  in- 
flame, and  will  even  cool  to  blackness  the  touching  part  of  the 
coal;  while  on  the  tardy  conductor  the  paper  becomes  almost 
immediately  as  hot  as  the  coal.  It  is  because  water  exposed  to 
the  air  cannot  be  heated  beyond  212°,  that  it  may  be  made  to 
boil  in  an  egg-shell  or  a  vessel  made  of  paper,  held  over  a  lamp, 
without  the  containing  substance  being  destroyed;  but  as  soon 
as  it  is  dried  up,  the  paper  will  burn  and  the  shell  will  be  cal- 
cined, as  the  solder  of  a  common  tinned  kettle  melts  under  the 
same  circumstances.  The  reason  why  the  hand  judges  a  cold 
liquid  to  be  so  much  colder  than  a  solid  of  the  same  tempera- 
ture is,  that  from  the  mobility  of  the  liquid  particles  among 
themselves,  those  in  contact  with  the  hand  are  constantly 
changing.  The  impression  produced  on  the  hand  by  very  cold 
mercury  is  almost  insufferable,  because  mercury  is  both  a  ready 
conductor  and  a  liquid.  Again,  if  a  finger  held  motionless  in 
water  feel  cold,  it  will  feel  colder  still  when  moved  about;  and 
a  man  in  the  air  of  a  calm  frosty  morning  does  not  experience  a 
sensation  nearly  so  sharp  as  if  with  the  same  temperature  there 
be  wind.  A  finger  held  up  in  the  wind  discovers  the  direc- 
tion in  which  the  wind  blows  by  the  greater  cold  felt  on  one 
side,  the  effect  being  still  more  remarkable,  if  the  finger  is  wet- 
ted. If  a  person  in  a  room  with  a  thermometer,  were  with  a 
fan  or  bellows  to  blow  the  air  against  it,  he  would  not  thereby 
lower  it,  because  it  had  already  the  same  temperature  as  the  air, 
yet  the  air  blown  against  his  own  body  would  appear  colder 
than  when  at  rest,  because,  being  colder  than  his  body,  the  mo- 


SPREADING  BV  CONDUCTION 

(ion  would  supply  heat-absorbing  particles  more  quickly.  In 
Hke  manner,  if  a  fan  or  bellows  were  used  against  a  thermome- 
ter hanging  in  a  furnace  or  hot  house,  the  thermometer  would 
suffer  no  change,  but  the  air  moved  by  them  against  a  person 
would  be  distressingly  hot,  like  the  blasting  sirocco  of  the  sandy 
deserts  of  Africa.  If  two  similar  pieces  of  ice  be  placed  in  a 
room  somewhat  warmer  than  ice,  one  of  them  may  be  made  to 
melt  much  sooner  than  the  other,  by  blowing  on  it  with  a  bel- 
lows. The  reason  may  here  be  readily  comprehended  why  a 
person  suffering  what  is  called  a  cold  in  the  head,  or  catarrh 
from  the  eyes  and  nose,  experiences  so  much  more  relief  on  ap- 
plying to  the  face  a  handkerchief  of  linen  or  cambric  than  one 
of  cotton: — it  is,  that  the  former  by  conducting  readily  absorbs 
the  heat  and  diminishes  the  inflammation,  while  the  latter,  by 
refusing  to  give  passage  to  the  heat,  increases  the  temperature 
and  the  distress.  Popular  prejudice  has  held  that  there  was  a 
poison  in  cotton. 

"  Heat  spreading  in  fluids  chiefly  by  the  motion  of  their 
particles."     (Read  the  Analysis,  page  13.) 

Owing  to  the  mobility  among  themselves  of  fluid  particles, 
heat  entering  a  fluid  any  where  below  the  surface,  by  dilating 
and  rendering  specifically  lighter  the  portion  heated,  allows 
the  denser  fluid  around  to  sink  down  and  force  up  the  rarer; 
and  the  continued  currents  so  established,  diffuse  the  heat 
through  the  mass  much  more  quickly  than  heat  spreads  by  con- 
duction in  any  solid. 

Count  Rumford's  experiments  led  him  at  first  to  conclude 
that  liquids,  but  for  this  carrying  process,  by  the  particles 
changing  their  place,  were  absolutely  impassable  to  heat.  A 
piece  of  ice  will  lie  very  long  at  the  bottom  of  water  which 
is  made  to  boil  at  the  top  by  the  contact  of  any  hot  body;  and 
when  it  at  last  melts,  Count  R.  believed  that  it  did  so  entirely 
from  the  heat  which  passed  downwards  through  the  sides  of  the 
vessel  containing  the  water.  But  an  ingenious  experiment  by  Dr. 
Murray  decided  the  question  differently.  He  made  a  vessel  of 
ice,  which,  of  course,  could  not  carry  downwards  any  heat  great- 


HEAT. 

er  than  32°,  as  ice  melts  at  that  degree;  and  having  put  into  it  a 
quantity  of  oil  at  32°,  the  bulb  of  a  thermometer  being  a  quar- 
ter of  an  inch  under  the  surface  of  the  oil,  he  placed  a  cup  of 
boiling  water  in  contact  with  the  surface  of  the  oil: — in  a  mi- 
nute and  a-half  the  thermometer  rose  nearly  a  degree,  and  in 
seven  minutes  it  rose  five  degrees,  beyond  which  it  did  not  go. 
The  heat  then  must  have  passed  downwards  through  the  liquid, 
proving  a  conducting  power; — unless,  indeed,  it  passed  by  ra- 
diation, as  explained  in  a  subsequent  page. 

The  internal  currents  or  circulation  produced  by  heat  in  fluid 
masses,  and  of  which  there  are  so  many  important  instances  in 
nature,  were  more  fitly  explained  in  the  chapter  on  hydrosta- 
tics and  pneumatics;  we  shall  here  therefore  allude  to  them 
very  shortly. 

Perhaps  the  best  experimental  illustration  of  the  subject  is 
the  placing  a  tall  glass  jar,  filled  with  water  in  which  small 
pieces  of  amber  are  floating  to  show  its  movements,  first  in  a 
warm  bath,  and  then  in  one  which  is  cold.  In  the  first  case, 
the  water  and  amber  near  the  outside  of  the  jar  where  they  are 
heated,  will  exhibit  a  rapid  upward  current,  while  in  the  cen- 
tre of  the  jar  they  will  form  an  opposite  or  downward  current. 
By  afterwards  placing  the  jar  in  a  cold  bath,  the  direction  of 
the  currents  will  be  reversed. 

It  is,  as  stated  in  the  former  volume,  this  heating  and  dilata- 
tion of  the  fluid  air  over  a  tropical  island  while  acted  upon 
during  the  middle  of  the  day  by  the  powerful  rays  of  the  sun, 
that  allows  the  colder  and  heavier  air  from  the  face  of  the 
ocean  around  to  press  inwards  upon  it  and  force  it  upwards  in 
the  atmosphere — the  cold  current  forming  the  delightful  sea- 
breeze  of  the  climate.  And  it  is  the  general  heating  of  tho 
air  over  the  whole  equatorial  belt  of  the  earth,  which,  render- 
ing it  specifically  lighter  than  the  air  nearer  the  poles,  allows 
this  to  assume  the  form  of  cool  trade  winds,  constantly  blow- 
ing towards  the  sun's  path,  and  pressing  upwards  the  hot  air, 
which  then  spreads  away  on  the  top  of  the  atmosphere  towards 
the  poles,  to  mitigate  the  severity  of  the  northern  and  south- 
ern cold.  In  the  watery  ocean  also  there  is  a  circulatory  mo- 
tion of  the  same  kind,  although  less  in  degree,  tending  to  dis- 


BY  CONDUCTION.  >\l 

tribute  heat  and  equalize  temperature,  and  contributing  to  pro- 
duce some  of  the  great  sea  currents  known  to  mariners. 

The  vertical  currents  produced  by  heat,  in  the  ocean,  and  in 
great  masses  of  water  generally,  preserve  in  and  over  them  a 
comparatively  uniform  temperate  freshness,  while  the  rocks 
and  soil  around  may  be  either  parched  under  a  burning  sun,  or 
bound  up  in  cold  many  degrees  below  freezing.     A  keen  frost 
chills,  and  soon  hardens  in  its  icy  grasp  the  surface  of  the 
ground;  but  of  water  similarly  exposed,  the  part  first  cooled 
descends  to  the  bottom  by  its  increased  density,  and  forces  up 
a  warmer  water  to  take  its  place;  this  in  its  turn  is  cooled  and 
descends,  and  a  continued  circulation  is  established,  so  that 
the  surface  cannot  become  ice  until  the  whole  mass,  of  what- 
ever depth,  has  been  cooled  down  to  its  greatest  density.     The 
very  deep  sea,  hence,  is  not  frozen  in  the  coldest  climates,  and 
in  temperate  climates  the  severest  winter  does  not  freeze  even 
a  deep  lake.    During  this  intestine  movement  in  the  water,  that 
which  ascends  to  the  surface  to  be  cooled,  by  losing  one  degree 
of  its  heat,  warms  nearly  five  hundred  times  its  bulk  of  air  one 
degree,  and  thus  tempers  remarkably  the  air  passing  over  it. 
Hence,  places  in  the  vicinity  of  the  sea  and  of  lakes  are  warmer 
in  winter  than  places  farther  inland,  although  nearer  the  equa* 
tor.     England  is  much  warmer  in  winter  than  central  Germa- 
ny, which  lies  south  of  England,  and  the  coasts  of  Scotland 
and  the  north  of  Ireland  are  warmer  than  London: — snow  ne* 
ver  lies  long  upon  these  coasts.    As  continental  or  inland  coun- 
tries have  thus  in  winter  an  extreme  of  cold,  so  have  they  ia 
summer  an  extreme  of  heat.     Water  admits  the  rays  of  the 
sun,  and  absorbs  the  heat  into  the  whole  thickness  of  its  mass^ 
(it  will  be  explained  afterwards  that  solar  heat  penetrates  trans- 
parent fluids  as  light  does,)  while  earth  retains  all  near  its  sur- 
face, and  is  therefore  heated  to  excess. 

The  ventilation  of  our  dwellings  and  halls  of  assembly  (as 
explained  in  vol.  i.)  is  owing  to  the  motion  produced  by  the 
changed  specific  gravity  of  air  when  heated.  The  air  which 
is  within  the  house  becomes  warmer  than  the  external  air,  and 
this  then  presses  in  at  every  opening  or  crevice  to  displace  it. 
The  ventilation  of  the  person  by  the  slow  passage  of  air  through 


33  BEAT. 

the  texture  of  our  clothing  is  a  phenomenon  of  the  same  kind; 
and  thicker  clothing  acts  chiefly  by  diminishing  the  passage. 
Hence,  an  oiled  silk  or  other  air  tight  covering  laid  on  a  bed,  has 
greater  influence  in  preserving  warmth  than  an  additional  blan- 
ket or  more.  From  the  part  of  bed-clothes  immediately  over 
the  person  there  is  a  constant  outward  oozing  of  warm  air, 
and  there  is  an  oozing  inward  of  cold  air  in  lower  situations 
around.  In  many  persons  the  circulation  of  the  blood  is  so 
feeble  that  in  winter,  they  have  great  difficulty  in  keeping  their 
feet  warm,  even  in  bed,  unless  with  the  assistance  of  a  bottle 
of  hot  water  or  some  such  means,  and  in  consequence  they  of- 
ten pass  sleepless  nights,  and  suffer  in  their  general  health.  At 
the  suggestion  of  the  author,  in  such  cases,  a  long  flexible  tube 
has  been  used, — as  of  spiral  wire  covered  with  leather  or  oiled 
silk,  by  which  persons  can  send  down  to  their  lower  extremi- 
ties their  hot  breath,  and  thus  supply  to  them  effectually  a  na- 
tural animal  warmth,  as  in  a  cold  day  they  warm  their  hands 
by  blowing  upon  them  through  their  gloves. 

The  power  of  fluids  to  diffuse  heat  being  due  thus  to  their 
power  of  carrying,  and  not  of  conducting  it,  the  consequence 
should  follow,  that  any  circumstance  which  impedes  the  inter- 
nal motion  of  the  fluid  particles,  should  diminish  the  diffusing 
power.  Accordingly,  we  find  that  fluids  in  general  transfer 
heat  less  readily  in  proportion  as  they  are  more  viscid.  Wa- 
ter, for  instance,  transfers  less  quickly  than  spirits;  oil  than 
water;  molasses  or  syrup  than  oil:  and  water  thickened  by 
starch  dissolved  in  it,  or  which  has  its  internal  motion  im- 
peded by  feathers  or  thread  immersed  in  it,  less  quickly  than 
where  it  is  pure  and  at  liberty.  Cooling  being  merely  a  mo- 
tion, the  reverse  of  heating,  it  is  influenced  by  the  same  law. 
Hence,  the  reason  why  soups,  pies,  puddings,  and  all  semifluid 
masses,  retain  their  heat  so  long,  so  much  longer  than  equal  bulks 
of  mere  fluid.  The  same  law  affords  explanation  of  the  facts, 
that  very  porous  masses  and  powders,  as  charcoal,  metal  filings, 
sawdust,  sand,  &c.  conduct  heat  more  slowly  than  denser  masses; 
their  interstices  being  filled  with  air,  which  scarcely  conducts 
heat,  and  which,  by  the  structure  of  the  substance,  has  no  free- 
dom of  motion  or  circulation  by  which  it  might  carry  tho 
heat 


SPREADING  BT  RADIATION.  33 

"  Heat  spreads  also,  partly,  by  being  shot  or  radiated  like 
light,  from  one  body  to  another,  through  transparent  me- 
dia or  space,  with  readiness  affected  by  the  material  and 
the  state  of  the  giving  and  receiving  surfaces."  (Read 
the  Analysis,  page  13.) 

If  a  heated  ball  of  metal  be  suspended  in  the  air,  a  hand 
brought  in  any  direction  near  to  it  will  experience  the  sensa- 
tion of  heat:  and  beneath  it  the  sensation  will  be  as  strong  as 
on  the  side,  although  the  heat  has  to  shoot  down  through  an 
opposing  current  of  air  approaching  the  heated  ball,  to  rise 
from  it,  as  explained  in  a  preceding  section.  A  delicate  ther- 
mometer substituted  for  the  hand  will  equally  detect  the  spread- 
ing heat,  and  if  held  at  different  distances,  will  prove  it  to  di- 
minish in  the  same  ratio  as  light  diminishes  in  spreading  from 
any  luminous  centre,  viz.  to  be  only  a  fourth  part  as  intense 
at  a  double  distance,  and  in  a  corresponding  proportion  for 
other  distances.  If  the  heated  body  be  enclosed  in  a  vacuum, 
a  thermometer  placed  near  it  will  still  be  affected  in  the  same 
manner.  If  a  screen  be  interposed  between  the  body  and  the 
thermometer,  the  latter  will  not  be  affected  at  all,  proving  the 
heat  to  spread  in  straight  lines.  Heat  when  diffusing  itself  in 
this  way,  to  distinguish  it  from  heat  passing  by  contact  or  com- 
munication, as  described  in  the  last  section,  is  called  radiant 
heat  or  caloric;  that  is  to  say,  spreading  in  rays  all  round  its 
source  as  light  spreads. 

Radiant  heat  resembles  light,  yet,  in  other  respects.  It  as 
rapidly  permeates  certain  transparent  substances,  and  its  course 
suffers  in  them  a  degree  of  the  bending,  termed  refraction  by 
opticians.  It  is  reflected  from  many  kinds  of  polished  surfaces, 
just  as  light  is  reflected  from  a  common  mirror;  and  many  such 
surfaces  directed  to  one  centre  (as  when  Archimedes  made  the 
sun  his  assistant  to  burn  the  Roman  ships)  or  a  single  concave 
surface,  having  its  own  centre  or  focus,  will  concentrate  heat 
just  as  it  does  light.  Its  motion  in  the  sun-beam  is  so  rapid  as, 
for  any  distance  at  which  men  can  try  the  experiment,  to  ap- 
pear instantaneous;  and  the  rays  of  heat  from  hot  iron  or  burn- 
ing charcoal,  concentrated  at  great  distances  by  suitable  mirrors, 
a  thermometer  as  quickly  a»  the  heat  of  the  setting  sun 
5 


BEAT. 

reflected  from  a  distant  window.  Although  light  and  heat  ars 
united  in  the  sun's  ray,  they  are  still  separable  by  our  glass 
prisms  or  lenses;  and  the  focus  of  heat  behind  a  burning-glass 
is  not  precisely  the  focus  of  light.  Heat  in  radiating  through 
air  does  not  warm  it,  and  is  not  affected  by  winds  or  any 
other  motion  of  the  air.  These  resemblances  in  the  phenomena 
of  light  and  heat  have  by  some  inquirers  been  held  to  prove 
that  the  two  classes  of  appearances  are  only  different  modifica- 
tions of  action  in  the  same  subtle  substance  or  ether. 

The  diffusion  of  heat  by  radiation,  as  it  takes  place  in  an  in- 
stant to  any  distance,  and  begins  whenever  there  is  any  inequa- 
lity of  temperature  between  bodies  exposed  to  each  other,  would 
produce  instant  balance  of  temperature  throughout  nature,  but 
that  heat  leaves  and  enters  bodies  with  readiness  depending  on 
their  internal  conducting  powers,  and  on  the  condition  of  their 
surfaces.  A  black  stone-ware  tea-pot,  for  instance,  will  radiate 
away  100  degrees  of  its  heat  in  the  same  time  that  a  pot  of  po- 
lished metal  will  radiate  12  degrees. 

Professor  Leslie  was  the  first  to  see  the  importance  of  inves- 
tigating this  subject,  and  he  had  the  merit  of  contriving  well- 
adapted  means,  and  of  detecting  many  of  the  important  facts. 
As  common  thermometers  are  not  sufficiently  delicate  to  deter- 
mine very  sudden  changes  of  temperature,  where  the  influence 
is  so  slight  as  in  many  cases  of  radiant  heat,  he  used  the  beau- 
tiful differential  thermometer,  contrived  by  himself,  in  con- 
junction with  concave  mirrors,  to  concentrate  the  heat  and  ac- 
cumulate its  energy.  Then  taking  as  his  heated  body  a  cubical 
tin  vessel  filled  with  boiling  water,  and  covering  it  successively 
with  plates  or  layers  of  different  substances,  and  with  different 
colours;  and  exposing  the  thermometer  to  it  under  all  the 
changes,  he  noted  the  number  of  degrees  which  the  thermome- 
ter rose  —  as  seen  in  the  table  which  here  follows,  and  thus  as- 
certained the  radiating  power  of  each  sort  of  covering. 


Lamp  black  -                           -     100 

Writing  paper  -                                   98 

Crown  glass    -  -                         90 

Ice  -      87 

Isinglass  75 


SPREADING  BY  RADIATION.  35 

Tarnished  lead        -  45° 

Clean  lead    .    -  -  19 

Iron  polished  -  15 

Tin  plate  -  12 

Gold,  silver,  and  copper  -  -  12 

He  next  reversed  the  experiments  by  using  his  hot  water  ves- 
sel always  in  the  same  state,  and  covering  the  thermometer  bulb 
with  the  different  substances  and  colours,  and  thus  he  ascer- 
tained that  their  comparative  absorbing  powers  were  very  near- 
ly proportioned  to  their  radiating  powers:  lamp  black,  for  in- 
stance, absorbed  or  was  heated  100°,  while  the  polished  metals 
absorbed  or  were  heated  only  12°,  and  so  for  the  others.  And, 
lastly,  the  absorbing  powers  being  likewise  an  indication  of  the 
opposite  or  reflecting  powers,  (for  if  a  body  absorb  any  given 
proportion  of  the  heat  which  falls  on  it,  it  must  reflect  the  re- 
mainder,) he  had  at  the  same  time  ascertained  the  reflective  or 
mirror  powers  of  the  bodies,  and  therefore  all  the  important 
points  respecting  radiant  heat  in  its  relation  to  the  substances 
between  which  it  passes. 

It  seems  paradoxical  that,  by  putting  a  clothing  of  a  thin  cot- 
ton or  woollen  fabric  upon  the  polished  tin  vessel,  the  heat 
should  be  received  by  it  or  dissipated  from  it  much  sooner  than 
if  the  vessel  were  naked,  but  such  is  the  fact.  And  metal  with 
a  scratched  or  roughened  surface  radiates  or  receives  much  more 
rapidly  than  if  polished. 

The  property  of  absorbing  heat  depends  much  upon  the  co- 
lour of  the  substance,  and  as  a  general  rule  the  dark  colours, 
viz.  those  which  absorb  most  light,  absorb  also  most  heat.-  Dr. 
Franklin  laid  pieces  of  cloth  of  different  colours  on  snow,  and 
during  a  given  period  in  which  the  sun  was  shining  on  them, 
he  noted  this  in  the  different  depths  to  which,  by  melting  the 
snow  which  was  under  them,  they  sunk.  Hence,  appears  the 
importance  of  having  a  white  dress  in  summer,  that  by  it,  with 
the  sun's  light,  the  heat  also  may  be  repelled;  and  a  white  dress 
in  winter  is  good,  because  it  radiates  little.  Polar  animals  have 
generally  white  furs.  White  horses  are  both  less  heated  in  the 
sun,  and  less  chilled  in  winter,  than  those  of  darker  hues. 

The  rate  of  cooling  in  bodies  must  be  influenced  by  all  the 


36 


HEAT. 


particulars  noted  above,  viz.  substance,  surface,  colour,  and  by 
the  excess  of  heat  in  the  cooling  body  as  compared  with  those 
around  it. 

The  concentrating  apparatus  used  for  experiments  on  the  ra- 
diation of  heat  consists  of  two  concave  tin  mirrors,  here  repre- 
sented at  a  and  b,  so  formed  and  placed  in  relation  to  each  other, 
that  all  the  rays  of  light  or  heat  issuing  from  the  focus  of  one; 


C     - 


as  at  c,  shall  be  collected  into  the  focus  of  the  other,  d.  A 
stand  under  one  focus,  e,  is  intended  to  support  the  body  giving 
out  or  receiving  heat,  and  a  stand  under  the  other,  d,  is  meant 
to  support  the  thermometer.  For  farther  explanation  of  the  ac- 
tion of  such  mirrors,  we  may  refer  to  what  was  said  of  the  con- 
centration of  sound  in  the  section  on  Acoustics,  or  to  what  fol- 
lows in  the  section  on  Optics,  on  the  concentration  of  light. 
The  general  rationale  of  such  facts  is,  that  heat,  light,  sound,  an 
elastic  ball,  &e.  reflected  from  any  point  of  a  surface,  returns, 
if  it  fall  perpendicularly  to  that  point,  in  the  same  line  by  which 
it  approached;  but  if  it  fall  obliquely,  or  one  side  of  the  perpen- 
dicular, it  returns  in  a  line  deviating  as  much  on  the  other  side. 
Now  the  surfaces  of  concave  mirrors  are  so  formed,  that  every 
ray  issuing  from  the  focus  shall,  when  reflected,  become  paral- 
lel to  every  other  ray — as  represented  by  the  dotted  lines  in  the 
figure;  and  it  is  the  property  of  a  similar  mirror  receiving  pa- 
rallel rays  to  make  them  all  meet  in  its  focus: — thus  any  influ- 
ence radiating  from  c,  will  be  again  collected  at  d.  The  pur- 
pose and  effect  of  such  mirrors  in  experiments  on  heat,  is  mere- 
ly to  concentrate  feeble  influences,  so  that  they  may  be  more 
accurately  estimated.  To  show  their  effect  and  mode  of  action, 
they  may  be  placed  exactly  facing  each  other  at  any  convenient 
distance,  and  then  a  hot  body  of  any  kind,  as  a  ball  of  metal  or 
a  canister  of  boiling  water,  being  left  in  one  focus  while  a  ther- 


RADIATION  OF  COLT),  07 

mometer  stands  in  the  other,  the  thermometer  will  instantly 
rise;  although,  if  left  in  any  intermediate  situation  nearer  to  the 
hot  body,  and  therefore  not  in  the  focus,  it  will  not  be  affected. 
If  burning  charcoal  be  placed  in  one  focus  and  a  readily  com- 
bustible substance  in  the  other,  the  latter  will  be  set  fire  to  at 
the  distance  of  thirty  feet  or  more. 

If  in  one  focus  of  the  mirror  apparatus  described  above,  there 
be  placed,  instead  of  the  canister  of  hot  water,  a  piece  of  ice, 
the  thermometer  in  the  other  focus  immediately  falls.  This 
has  been  called  the  radiation  of  cold,  and  persons  were  at  one 
time  disposed  to  think  that  it  proved  cold  to  have  a  positive  ex- 
istence distinct  from  heat.  The  case,  however,  is  merely  that 
the  thermometer  happens  then  to  be  the  hotter  body  in  one  fo- 
cus of  the  mirrors,  placed  in  relation  with  a  colder  body,  the 
ice,  in  the  other,  and  consequently  by  the  law  of  equable  diffu- 
sion, it  must  share  its  heat  with  the  ice,  and  will  fall.  The  mir- 
rors in  any  case  have  merely  the  effect,  by  preventing  the  spread- 
ing and  dissipation  of  the  radiant  heat  from  either  focus,  except 
towards  the  other,  of  making  two  distant  bodies  act  upon  each 
other  as  if  they  were  very  near.  All  the  heat  that  seeks  to  ra- 
diate from  the  thermometer,  d,  in  the  direction  of  the  large  sur- 
face of  the  mirror  b,  if  not  met  by  an  equal  tension  of  force  of 
temperature  in  the  other  mirror  or  focus  to  which  they  are  di- 
rected at  a  and  c,  will  radiate  away  to  c,  and  become  deficient 
at  d.  Some  inquirers  have  believed  that  heat  was  constantly 
radiating  in  exchange  from  substance  to  substance  (as  light  ra- 
diates constantly  between  opposed  bodies,)  only  more  copiously 
from  one  side,  if  the  temperature  of  that  was  higher:  others 
have  held  that  the  movement  only  took  place  when  the  balance 
of  temperature  was  destroyed;  and  this  is  the  simplest  view. 

There  is  a  remarkable  difference  in  one  respect  between  the 
heat  of  the  sun  and  tjiat  radiated  from  any  other  source,  riz. 
that  the  first  passes  through  air,  glass,  water,  and  transparent 
bodies  generally,  very  readily;  while  the  latter,  although  not 
obstructed  by  air,  is  almost  totally  intercepted  or  absorbed  in 
passing  through  any  of  the  other  substances  named.  In  our 
drawing-rooms  it  is  common  to  have  ornamented  glass  fire- 
screens, which,  while  they  allow  the  light  to  pass,  defend  the 


S3  HEAT. 

face  from  the  heat:  but  all  persons  know  that  the  heat  of  the 
sun  beams,  as  well  as  their  light,  enters  our  green  houses 
through  the  glass  which  covers  them.  A  glass  screen  inter- 
posed between  the  concave  mirrors  in  the  apparatus  above  de- 
scribed, destroys  almost  entirely  the  effect  of  the  heated  body 
in  one  focus,  on  the  thermometer  in  the  other,  and  the  trifling 
effect  really  produced  has  appeared,  to  some  to  be  owing  to  the 
heat  first  absorbed  by  the  screen  on  one  of  its  sides,  and  then 
radiated  from  the  other.  This  conclusion  seemed  to  be  sup- 
ported by  the  fact  that  screens  of  metal  or  of  glass,  covered 
with  lamp  black,  paper,  &c.,  allow  transmission  nearly  in  pro- 
portion to  their  several  absorptive  and  radiant  powers.  More 
careful  experiments,  however,  have  been  held  to  prove  that  a 
small  portion  of  the  heat  is  suddenly  radiated  through  the  glass. 
A  glass  mirror  reflects  the  light  of  a  fire,  but  at  first  retains  all 
the  heat,  and  only  radiates  it  afterwards  as  a  hot  body. 

The  doctrines  of  radiant  heat  make  us  aware  of  the  importance 
of  having  vessels  of  polished  metal  for  containing  liquids  or  any 
thing  which  we  desire  to  keep  warm;  hence  tea  and  coffee  pots, 
dishes  for  soup,  &c.  should  be  polished.  As  a  black  earthen  tea- 
pot loses  heat  by  radiation  nearly  in  proportion  to  the  number 
100,  while  one  of  silver  or  other  polished  metal  loses  only  as 
12,  there  will  be  a  corresponding  difference  in  their  aptitude  for 
extracting  the  virtues  of  any  substance  infused  in  them.  Pipes 
for  the  conveyance  of  steam  or  hot  air,  if  left  naked,  should  be 
of  polished  metal;  but  after  arriving  at  the  place  where  they 
have  to  give  out  their  heat,  their  surface  should  be  blackened 
and  rough.  A  coat  of  polished  mail  is  not  a  cold  covering.  A 
mirror  intended  to  reflect  heat  should  be  of  highly  polished  me- 
tal: and  such  is  the  interior  of  a  screen  placed  behind  roasting 
meat.  A  fireman's  mask  is  usually  covered  with  tin  foil.  It 
is  of  advantage  that  the  bottom  of  a  tea  kettle  or  other  cooking 
vessel  be  black,  because  the  bottom  has  to  absorb  heat,  but  the 
top  should  be  polished  because  it  has  to  confine. 

The  interesting  phenomenon  of  dew  was  not  at  all  under- 
stood until  lately,  since  the  laws  of  radiant  heat  have  been  in- 
vestigated. At  sun-rise  in  particular  states  of  the  sky,  every 
blade  of  grass  and  leaflet  is  found,  not  wetted  as  if  by  a  shower^ 


DEW,  G# 

but  studded  with  a  row  of  distinct  globules  most  transparent 
and  beautiful,  bending  it  down  by  their  weight,  and  falling  like 
pearls  when  the  blade  is  shaken.  These  are  formed  in  the 
course  of  the  night  by  a  gradual  deposition  on  bodies  rendered 
by  radiation  colder  than  the  air  around  them,  of  the  moisture 
which  rises  invisibly  from  water  surfaces  into  the  air  during  the 
heat  of  the  day.  In  a  clear  night  the  objects  on  the  surface  of 
the  earth  radiate  heat  upwards  through  the  air  which  impedes 
not,  while  there  is  nothing  nearer  than  the  stars  to  return  the 
radiation;  they  consequently  soon  become  colder,  and  if  the  air 
around  has  its  usual  load  of  moisture,  part  of  this  will  be  depo- 
sited on  them,  exactly  as  the  invisible  moisture  in  the  air  of  a 
room  is  deposited  on  a  cold  bottle  of  wine  when  first  brought 
from  the  cellar.  Air  itself  seems  not  to  lose  heat  by  radiation. 
A  thermometer  placed  upon  the  earth  any  time  after  sun-set  until 
sun-rise  next  morning,  generally  stands  considerably  lower  than 
another  suspended  in  the  air  a  few  feet  above  it;  owing  to  the 
radiation  of  heat  upwards  from  the  earth,  while  the  air  remains 
nearly  in  the  same  state.  During  the  day,  while  the  sun  shines, 
the  earth  is  much  warmer  than  the  air.  The  reason  why  the 
dew  falls,  or  forms  so  much  more  copiously  upon  the  soft  spon- 
gy surface  of  leaves  and  flowers,  where  it  is  wanted,  than  on 
the  hard  surface  of  stones  and  sand,  where  it  would  be  of  no 
use,  is  the  difference  of  their  radiating  powers.  There  is  no 
state  of  the  atmosphere  in  which  artificial  dew  may  not  be  made 
to  form  on  a  body,  by  sufficiently  cooling  it,  and  the  degree  of 
heat  at  which  it  begins  to  appear  is  called  the  dew  point,  and  is 
an  important  particular  in  the  meteorological  report  of  the  day. 
In  cloudy  nights  heat  is  radiated  back  from  the  clouds,  and  the 
earth  below  not  being  so  much  cooled,  the  dew  is  scanty  or  de- 
ficient. On  the  contrary,  when  uninformed  persons  would  least 
expect  the  dew,  viz.  in  warm  very  clear  nights,  and  perhaps 
when  the  beautiful  moon  invites  to  walking,  and  music  adds  its 
charm,  as  in  some  of  the  evenings  of  autumn  with  the  harvest 
moon  and  harvest  occupations — then  is  the  dew  more  abundant, 
and  the  danger  greater  to  delicate  persons  of  taking  harm  by 
walking  among  the  grass. 


40  HEAT. 

"  Heat  by  entering  bodies  expands  them,  and  through  a 
range  which  includes  as  three  successive  stages  the  forms 
of  solid,  liquid,  and  air,  or  gas;  becoming  thus,  in  nature^ 
the  grand  antagonist  and  modifier  of  the  effects  of  that 
attraction  which  holds  corporeal  atoms  together,  and 
which,  if  acting  alone,  ivould  reduce  the  whole  material 
universe  to  one  solid  lifeless  mass.  (Read  the  Analysis, 
page  13.) 

If  an  experimenter  take  a  body  which  is  as  free  from  heat  as 
man  can  procure  a  body — a  bar  of  solid  mercury,  for  instance, 
as  it  exists  in  a  polar  winter;  and  if  he  then  gradually  heat  such 
body,  to  whatever  extent,  it  will  acquire  an  increase  of  bulk 
with  every  increase  of  temperature:  first,  there  will  be  simple 
enlargement  or  expansion  in  every  direction;  then  the  mass  will 
in  addition  be  softened;  then  it  will  be  melted  or  fused,  that  is 
to  say,  in  the  case  supposed,  the  solid  bar  will  be  reduced  to 
the  state  of  liquid  mercury,  with  the  cohesive  attraction  of  the 
atoms  nearly  overcome;  if  the  mass  be  still  farther  heated,  the 
atoms  will  be  repelled  from  each  other  to  much  greater  dis- 
tances, constituting  then  a  very  elastic  fluid  called  an  air  or  gas, 
many  hundred  times  more  bulky  than  the  same  matter  in  the 
solid  or  liquid  state,  and  capable  of  forcibly  distending  an  ap- 
propriate vessel  as  common  air  distends  a  bladder;  susceptible, 
moreover,  of  dilating  indefinitely  farther,  by  farther  additions  of 
heat,  or  by  diminution  of  the  atmospheric  pressure,  against 
which  it  had  to  rise  during  its  formation.  A  subsequent  remo- 
val of  the  heat  will  cause  a  corresponding  progress  of  contrac- 
tion, and  the  various  conditions  or  forms  of  the  substance  above 
enumerated,  will  be  reproduced  in  a  reverse  order,  until  the  so- 
lid mass  again  appear. 

What  is  thus  true  of  mercury  is  proved  by  modern  chemical 
art  to  be  true  also  of  all  the  ponderable  elements  of  our  globe, 
and  of  many  of  the  combinations  of  these  elements, — as  water, 
for  instance,  familiarly  known  in  its  three  forms  of  ice,  water, 
and  steam;  although  compound  substances  generally,  by  great 
changes  of  temperature  are  decomposed  into  their  elements. 

A  student  might  at  'first  have  difficulty  in  believing  that  the 
beautiful  variety  of  solid,  liquid,  and  air  found  among  natural 


EXPANSION  AND  CHANGE  OP  FORM.  41 

bodies,  could  depend  upon  the  quantities  of  heat  in  them,  be- 
cause these  forms  are  all  seen  existing  at  the  same  temperature, 
but  he  afterwards  learns  that  each  substance  has  its  peculiar  re- 
lation or  affinity  to  heat,  and  that  hence,  while  at  the  medium 
temperature  of  the  earth,  some  bodies  contain  so  little  as  to  be 
solids — like  the  metals,  stones,  earths,  &c. ;  others  have  enough 
to  be  liquids — as  mercury,  water,  oils,  &c. ;  and  others  enough 
to  be  airs — as  oxygen,  nitrogen,  hydrogen,  &c.  Men,  until 
better  informed,  are  prone  to  deem  the  states  in  which  bodies 
are  most  frequently  observed,  to  be  the  natural  states  of  such 
bodies;  and  the  Indian  king  but  reasoned  in  a  usual  way,  who 
held  the  Dutch  navigators,  newly  arrived  on  his  shores,  to  be 
gross  imposters,  when  they  said  that  in  their  country,  at  one 
time  of  the  year,  water  became  so  hard  that  they  could  walk 
upon  it,  and  drive  their  carriages  upon  it,  and  shape  it  into 
solid  blocks.  All  persons  err  like  this  king,  who  in  thinking 
of  the  different  substances  known  to  «them,  regard  their  acci- 
dental state  as  to  the  cohesion  of  particles — which  state  is 
really  dependent  on  the  temperature  of  the  bodies,  and  there- 
fore on  the  particular  planet  or  situation  on  the  planet  where 
they  are  found,  to  be  in  them  an  essential  natural  character. 
As  well  might  a  person  who  had  never  seen  silk,  but  as  a  de- 
licate gauze  or  satin  enveloping  some  lovely  human  form,  re- 
fuse to  recognise  it  in  the  unsightly  coil  of  the  worm  which 
produces  it. 

The  degrees  in  a  general  scale  of  temperature  at  which  the 
most  important  substances  in  nature  change  their  states  from 
solid  to  liquid,  or  from  liquid  to  air,  will  be  noted  in  a  future 
page.  Here  we  have  only  to  remark,  that  the  differences  are 
very  great.  Mercury  melts  at  about  80°  below  the  melting 
point  of  ice,  and  porcelain  at  about  30,000°  above.  There  are 
some  substances  which  require  so  high  a  temperature  for  their 
fusion  or  for  their  conversion  into  gas,  that  human  art  has  dif- 
ficulty, or  even  finds  it  impossible,  to  produce  the  changes  by 
simple  concentration  of  heat;  but  all  such  are  readily  soluble 
in  some  other  substance,  possessing  already  the  form  of  liquid 
or  air:  as  when  gold  and  platinum  are  dissolved  in  nitro-muri- 
atic  acid — flint  in  the  fluoric  acid, — carbon  in  hydcogen  gas. 

6 


42  HEAT. 

Now  many  persons  may  not  have  reflected  that  the  dissolving 
a  solid  in  any  fluid  menstruum  is  merely  another  mode  of  melt- 
ing it  by  heat;  yet  this  is  the  truth,  for  the  menstruum  is  itself 
fluid,  only  because  of  the  much  heat  which  it  contains,  and  in 
dissolving  the  more  obdurate  substance,  it  does  so  merely  be- 
cause its  attraction  for  the  substance  brings  the  particles  into 
union  with  the  heat  which  already  exists  in  itself.  Heat  then 
is  the  only  and  universal  solvent.  Its  influence  is  interestingly 
seen  in  the  fact,  that  a  fluid  when  heated  can  dissolve  much 
more  of  a  solid  than  when  cold.  Water  while  hot  keeps  dis- 
solved twice  as  much  of  many  salts  as  it  can  when  its  temper- 
ature has  fallen. -^-There  are  again  in  nature  many  substances 
having  such  an  affinity  for  heat,  that  until  lately  they  have 
only  been  known  as  airs;  and  even  in  the  present  advanced 
state  of  art,  they  cannot  by  any  degree  of  mere  cooling  be  re- 
duced to  the  liquid  or  solid  form;  yet  all  such,  when  pressure  is 
added  to  the  cooling,  OB  when  the  chemical  attraction  for  them 
of  some  other  substances  which  already  exist  in  the  liquid  or 
solid  state,  is  made  to  co-operate,  may  be  reduced.  An  in- 
stance is  afforded  by  oxygen,  when  made  part  of  a  liquid  acid, 
or  of  a  solid  ore. 

Of  solids,  some  on  receiving  heat  become  soft  before  they 
are  liquified,  as  pitch,  glue,  iron,  &c. ;  others  change  com- 
pletely at  once,  as  ice  in  becoming  water;  and  some  pass  at 
once  to  the  state  of  air,  without  therefore  having  assumed  at 
all  the  intermediate  state  of  liquid — -they  are  sublimed,  as  it 
is  called,  and  on  cooling  again  may  be  caught  in  a  powdery 
state,  as  is  seen  in  that  form  of  sulphur,  or  of  benzoin,  termed 
the  flower  of  the  substance.  Of  the  latter  class  also  are  cam- 
phor, arsenic,  corrosive  sublimate,  and  the  substance  called 
iodine,  which  last,  from  the  state  of  rich  ruby  crystals,  on  be- 
ing heated  becomes  at  once- a  dense  transparent  gas  of  the  same 
hue,  and  in  cooling  resumes  its  crystalline  form. 

The  reader  having  arrived  at  this  place,  may  peruse  again 
with  advantage  five  pages  of  vol.  i.  (between  pages  57  and  64 
in  the  different  editions)  which  treat  of  the  influence  of  heat 
9 n,  tlie  constitution  of 


CAPACITY  FOR  HEAT.  43 

"  Each  particular  substance,  according  to  the  nature,  prox- 
imity, fyc.  of  its  ultimate  particles,  takes  a  certain  quan- 
tity of  heat  (said  to  mark  its  CAPACITY)  to  produce  in  it  a 
given  change  of  temperature  or  calorific  tension."  (Read 
the  Analysis,  page  13.) 

A  pound  of  water,  for  instance,  to  raise  its  temperature  one 
degree,  takes  thirty  times  as  much  heat  as  a  pound  of  mercu- 
ry. This  may  be  proved  in  various  ways.  First,  if  the  heat 
be  derived  from  any  uniform  source,  the  water  must  remain 
exposed  to  it  thirty  times  as  long  as  the  mercury.  Second,  if 
both  substances,  after  being  equally  heated,  be  placed  in  ice 
until  cooled  to  the  freezing  point,  the  heat  which  escapes  again 
from  the  water  will  melt  thirty  times  as  much  ice  as  that  which 
escapes  from  the  mercury.  Third,  when  a  pound  of  hot  wa- 
ter is  mixed  with  a  pound  of  cold  mercury,  instead  off  the  two 
becoming  of  a  middle  temperature,  as  is  the  case  when  equal 
quantities  of  hot  and  cold  water  are  mixed — every  degree  of 
heat  lost  by  the  one  becoming  just  a  degree  gained  by  the 
other — the  pound  of  hot  water,  by  giving  up  one"  degree  to  the 
pound  of  cold  mercury,  raises  the  temperature  o'f  the  latter 
thirty  degrees;  and  in  the  same  proportion  for  other  differ- 
ences:— or,  on  reversing  the  experiment;  a  pound  of  hot  mer- 
cury will  be  cooled  thirty  degrees  by  warming  a  pound  of  wa- 
ter one  degree. 

Now  each  particular  substance  in  nature,  just  as  water  or 
mercury,  has  its  peculiar  capacity  for  heat;  and  experiments- 
made  by  the  modes  of  mixture  and  of  melting  ic'e  above  de- 
scribed have  led  to  the  construction  of  tables  which  exhibit  the 
relations.  The  following  short  table  is  an  abstract  of  these, 
showing  the  comparative  capacities  of  equal  weights  of  some 
common  substances.  Water,  for  reasons  of  convenience,  has 
been  chosen  as  the  standard  of  comparison.  It  appears,  then, 
that  a  pound  of  hydrogen  gas  takes  about  twenty  times  more 
heat  to  produce  in  it  a  given  change  of  temperature  than  a. 
pound  of  water,  while  a  pound  of  gold  takes  about  twenty 
times  less,  and  therefore  four  hundred  times  less  than  the  hy- 
drogen. The  figures  in  the  table,  by  marking  the  comparativ6 
capacities  for  heat  of  various  substances,  necessarily  indicate 


44  HEAT. 

also  the  comparative  quantities  of  ice  which  would  be  melted 
by  equal  weights  of  the  substances  in  cooling  through  an  equal 
number  of  degrees.  A  pound  of  water,  the  standard,  must 
cool  140  degrees,  that  is,  must  give  up  140  degrees  of  its  heat 
to  melt  one  pound  of  ice. 

Gases. 

Hydrogen  -  21$ 

Atmospheric  air  1$ 

Carbonic  acid  gas  -  -                   IT 

Common  steam  -  -         -         -         1$ 


Liquids. 
Solution  of  carbonate  of  ammonia       -         2 

^  To 


Alcohol 1  l 


Water     - 
Milk 
Olive  oil 
Linseed  oil 
Sulphuric  acid 
Quicksilver 


Solids. 


Ice  T9ff 

Wheat  .....  £ 

Charcoal  .  $ 

Chalk  -  * 

Glass  i- 

Iron  -  § 

Zinc  TV 

Gold  •         .  -        -         3V 

We  may  remark  here  that  some  late  researches,  by  an- 
other mode  of  trial,  make  the  capacity  of  air  to  be  only  a 
quarter  that  of  water,  although  in  the  preceding  table  it  ap- 
pears to  be  one  and  three-quarters.  Now  as  the  other  aeriform 
fluids  have  been  compared  with  water  through  the  medium  of 
atmospheric  air,  if  there  be  an  error  with  respect  to  this,  it 


CAPACITY  FOR  HEAT.  45 

must  run  through  all  the  figures  noting  the  capacity  of  other 
aeriform  substances. 

If  we  seek  a  reason  or  reasons  why  there  should  be  among 
bodies  the  differences  of  capacity  here  stated,  the  circumstances 
chiefly  calling  attention  are  the  following.  First,  equal  weights 
of  the  various  substances  have  very  different  bulks  or  volumes, 
and  therefore  have  different  room  in  which  the  heat  may  lodge. 
Mercury,  for  instance,  is  only  one-fourteenth  part  as  bulky  as 
water.  That  the  bulk,  however,  is  not  the  only  influencing 
circumstance  appears  in  the  fact,  that  mercury  while  having 
one-fourteenth  of  the  bulk  of  water,  has  only  one-thirtieth  of 
the  capacity.  Second,  in  equal  bulks  of  different  substances, 
the  space  may  be  more  completely  occupied  by  the  particles  of 
one  than  of  another — as  by  the  particles  of  mercury  than  by 
those  of  water.  But  as  the  facts  are  not  fully  accounted  for, 
even  by  both  of  these  circumstances,  we  must  seek  explana- 
tion in  a  third,  viz.  a  difference  in  the  ultimate  particles  of  bo- 
dies affecting  their  relations  to  heat. 

First,  The  influence  of  bulk  or  volume,  in  determining  the 
capacity  for  heat,  is  proved  by  the  facts  stated  in  the  preceding 
table,  and  by  many  others.  In  the  table,  for  instance,  it  is 
seen  that  hydrogen  and  the  gases  generally,  with  their  great 
comparative  bulk,  have  also  great  capacity;  that  liquids  have 
less  capacity  than  gases;  that  solids  have  less  than  liquids — but 
the  capacity,  as  already  stated,  is  not  in  strict  proportion  to 
bulk;  for  hydrogen,  which  is  many  thousand  times  more  bulky 
than  an  equal  weight  of  water,  has  only  twenty-one  times  the 
capacity.  Again,  if  any  body  whatever  be  suddenly  com- 
pressed into  less  bulk,  heat  will  issue  from  it  as  if  squeezed 
out.  Thus  iron  or  other  metal  suddenly  condensed  by  the 
heavy  blow  of  a  hammer,  is  thereby  rendered  hotter,  and  the 
expelled  heat  will  gradually  spread  from  it.  Because  water 
and  spirit,  on  being  mixed,  occupy  less  space  than  when  sepa- 
rate, there  is  from  the  mixture  a  corresponding  discharge 
of  heat.  But  the  truth  is  most  remarkably  exemplified  in  airs 
or  gases,  owing  to  their  great  range  of  elasticity.  They  may 
be  condensed  or  dilated  a  hundred  fold  or  more,  and  there  will 
be  a  simultaneous  concentration  or  diffusion  of  their  heat,  that 


46  HEAT. 

is  to  say,  the  production,  in  the  space  occupied  by  them,  of  in- 
tense heat  or  cold.  The  heat  of  air  just  condensed,  or  the  cold 
of  that  which  has  just  expanded,  is  much  greater  than  even  the 
most  delicate  thermometer  can  indicate,  for  there  is  so  little  heat 
altogether  even  in  a  considerable  volume'of  air,  that  the  mass 
of  a  mercurial  thermometer,  although  absorbing  a  great  part  of 
it,  would  be  little  affected.  The  extent,  however,  of  the 
change  of  temperature  is  seen  in  the  facts,  that  by  the  sudden 
condensation  of  air  we  may  inflame  tinder  immersed  in  it,  and 
by  allowing  air  suddenly  to  expand,  we  may  .convert  any  wa- 
tery vapour  diffused  through  it  into  ice  or -snow.  Nay,  air, 
containing  carbon  in  perfect  solution,  as  is  true  of  the  common 
coal  gas,  if  first  condensed  to  expel  heat,  and  then  allowed  sud- 
denly to  expand,  will  be  so  cooled -that  the  carbon  will  be  se- 
parated like  a  black  cloud,  as  snow  is  separated  in  the  case  be- 
fore described.  The  cold  which  separates  or  freezes  carbon 
from  a  gas  holding  it  in  solution,  is  perhaps  the  most  intense 
which  art  can  produce.  It  might  be  expected  that  air  suddenly 
compressed  into  half  its  previous  volume,  should  become  just 
twice  as  hot  as  before,  or  if  suddenly  dilated  to  double  volume, 
should  be  only  half  as  hot,  thus  enabling  us  to  ascertain  the 
whole  quantity  of  heat  contained  in  it;  but  the  facts  are  not 
so;  the  temperature  changes,  near  the  middle  degrees  of  the 
scale  at  least,  much  less  than  the  density.  Air  in  doubling  its 
volume  from  a  common  density,  becomes  colder  only  by  about 
50°  of  Fahrenheit's  thermometer. 

The  different  capacity  for  heat  of  air  in  different  states  of  di- 
latation, produces  effects  of  great  importance  in  nature  as  well 
as  in  the  arts — thus, 

On  the  surface  of  the  earth,  near  the  sea-shore,  the  air  of  the 
atmosphere  has  a  certain  density  (a  cubic  foot  weighs  about  one 
ounce  and  a  quarter)  dependent  on  the  weight  and  pressure  of 
the  superincumbent  mass;  but  on  a  mountain  top  15,000  feet 
high,  as  half  the  mass  of  the  atmosphere  is  below  that  level, 
(see  "  Pneumatics,")  the  air  is  bearing  but  half  the  pressure 
and,  consequently,  has  twice  the  volume  of  an  equal  quantity 
of  air  at  the  sea-side,  and  a  temperature,  consequently,  many 
degrees  inferior;  and  the  air  which  is  at  any  time  on  the  moun- 


CAPACITY  FOR  HEAT.  47 

tain-top,  may  have  been  recently  before  on  an  adjoining  plain 
or  shore,  and  in  gradually  climbing  the  mountain-side,  as  a 
wind,  it  must  have  been  gradually  expanding  and  cooling  in 
proportion  to  the  diminishing  pressure.  It  is  found  that  air, 
at  first  rising  from  the  sea-shore,  becomes  one  degree  colder 
for  about  200  feet  of  perpendicular  ascent,  and  altogether  about 
50°  colder  in  rising  15,000  feet;  so  that  at  this  latter  elevation, 
water  is  frozen  even  near  the  equator,  where  the  temperature 
of  low  plains  is  at  least  30°.  It  thus  appears,  that  if  a  man 
could  travel  with  the  wind  so  as  to  remain  always  surrounded 
by  the  same  air,  he  might  begin  his  journey  with  it  from  the 
summer  vineyards  of  the  Rhine,  might  soon  after  find  it  the 
piercing  blast  of  the  Alpine  summits;  and  again,  a  little  after, 
without  any  change  having  occurred  in  the  absolute  quantity 
of  its  heat,  might  feel  it  as  the  warm  breath  of  the  flowers  on 
the  plains  of  Italy. 

The  explanation  is  here  ready,  of  why  very  elevated  moun- 
tains in  all  parts  of  the  earth  are  hooded  in  perpetual  snows. 
We  have  just  said,  that  even  at  the  equator,  where  the  average 
temperature  near  the  sea  is  84°,  water  will  be  frozen  when  car- 
ried, to  an  elevation  of  15,000  feet.  A  line,  therefore,  traced 
on  a  mountain  at  this  level,  would  divide  the  portion  of  it  des- 
tined to  sleep  under  lasting  ice  and  snow  from  the  portion  be- 
low covered  with  green  herbage.  This  line,  wherever  found, 
is  called  the  snow  /me,  or  line  of  perpetual  congelation.  At 
the  equator  it  is  high  in  the  atmosphere,  because  there  is  a  dif- 
ference of  about  50°  between  the  average  temperature  of  the 
country  and  the  freezing  point  of  water,  viz.  the  difference  be- 
tween 84°  and  32°,  and  an  elevation  of  15,000  feet  corresponds 
to  this  difference;  but  in  a  progress  towards  the  poles,  the  line 
is  met  with  gradually  nearer  to  the  earth,  as  the  difference  in 
question  is  less.  In  Switzerland  it  is  at  6,500  feet  above  the 
sea;  in  Norway,  it  is  below  5,000.  With  respect  to  the  line 
of  congelation,  it  is  farther  to  be  remarked,  that  in  tropical 
countries,  because  the  temperature  of  the  air  is  nearly  uniform 
during  the  whole  year,  the  line  or  limit  of  frost  and  snow  is 
distinct  and  unvarying,  that  is  to  say,  is  narrow,  particularly 
where  the  acclivity  is  considerable;  but,  in  countries  to  the 


48  HEAT. 

north  and  south,  which  experience  strong  contrast  of  summer 
and  winter,  the  line  becomes  broad  and  less  evident;  because 
in  the  hot  season  much  snow  is  melted,  or  half  melted,  above 
what  may  be  called  an  average  line,  while  in  winter  much  snow 
and  ice  are  accumulated  below  this,  to  be  melted  again  when 
summer  returns. 

In  the  breadth  of  the  line  of  congelation  for  changeable  cli- 
mates, we  have  the  reason  of  the  formation  of  what  are  called 
glaciers  around  snow-capped  mountains  situated  in  such  cli- 
mates, and  around  such  only.  The  snow  near  the  upper  part  of 
the  broad  line  having  been  only  softened  or  half  thawed  in  the 
preceding  summer,  becomes  in  winter,  almost  as  solid  as  ice, 
and  in  the  succeeding  summer  vast  masses  of  this,  detached  by 
the  action  of  the  sun,  and  of  the  central  heat  of  the  earth 
spreading  outwards,  and  loaded  with  more  recent  accumulation 
of  snow,  are  constantly  falling  down  into  the  valleys  beneath; 
where,  being  accumulated,  and  the  crevices  filled  up  with  snow 
or  with  water,  which  hardens  to  ice,  they  form  at  last  the  huge 
glaciers  or  seas  of  ice — mers  de  glace,  which  render  certain 
regions  so  remarkable.  The  falling  of  such  masses  (called  in 
Switzerland  avalanches,}  is  what  renders  the  ascent  to  snow- 
clad  mountains  terrific  and  dangerous.  Around  Mont  Blanc, 
in  the  awful  solitudes  of  the  elevated  valleys,  the  avalanches 
are  thundering  down  almost  without  interruption  during  the 
whole  summer — in  which  season  only  the  attempt  to  ascend 
the  mountain  can  be  made,  and  a  pistol-shot,  or  any  consider- 
able agitation  of  the  air,  may  suffice  to  set  loose  masses  that 
will  sweep  away  a  whole  convoy.  Beneath  glaciers  there  is 
always  going  on  a  melting  of  that  part  of  the  ice  which  is  in 
contact  with  the  earth,  and  hence  a  stream  of  water  constantly 
issues  from  the  bed  of  every  glacier.  These  streams,  in  Swit- 
zerland, are  the  beginnings  of  the  magnificent  rivers,  the  Rhine 
and  Rhone.  Like  the  avalanches  breaking  loose  in  summer 
among  the  mountains,  there  are  in  the  polar  seas  vast  masses 
of  ice  detached  from  the  shores,  and  which  afterwards  move 
into  warmer  seas  to  be  melted.  These  often  become,  to  the 
arctic  bear,  rafts,  on  which,  to  his  surprise,  he  finds  himself 
voyaging  into  new  latitudes,  to  be  left  at  last  adrift  in  the  wide 
ocean  when  his  ship  has  vanished  from  beneath  him. 


CAPACITY  OP  BODIES.  49 

Although  the  proofs  are  not  so  immediately  apparent,  the 
line  of  congelation  exists  as  truly  every  where  in  the  open 
sky,  over  sea  and  plains,  as  where  there  are  mountain  heights 
to  wear  its  livery;  and  considerably  below  the  line,  the  cold, 
aided  by  electrical  agency,  is  sufficient  to  produce,  in  the  form 
of  mist  or  clouds,  a  deposition  from  the  air  of  the  watery  va- 
pour contained  in  it.     There  is  thus  in  nature  an  admirable 
provision  to  shade  the  earth  at  proper  times  from  the  too  pow- 
erful rays  of  the  sun,  or  to  supply  rain  as  wanted,  without  the 
transparency  of  the  inferior  regions  of  the  atmosphere  being 
much  affected.      As  the  watery  vapour,  rising  from  sea  or 
lake,  and  invisibly  diffused  in  the  atmosphere,  can  only  reach 
to  the  height  where  the  cold  is  great  enough  to  condense  it, 
the  clouds  may  in  general  be  regarded  as  the  top  of  that  at- 
mosphere of  watery  vapour,  or  aeriform  water,  which  is  al- 
ways mixed  more  or  less  with  the  atmosphere  of  mere  air;  and 
as  the  quantity  of  watery  vapour  which  can  exist  invisibly  in 
a  given  space,  depends  altogether  on  the  intensity  of  heat  pre- 
sent, the  clouds  in  a  humid  atmosphere  will  be  low,  and  in  a 
dry  atmosphere  will  be  high,  or  there  may  be  none.    An  aero- 
naut mounting  in  his  balloon  through  a  clear  sky,  often  ap- 
proaches a  dense  cloudy  stratum  to  plunge  into  it,  and  for  a 
time  to  be  surrounded  with  gloom  almost  of  night;  the  face  of 
earth  being  hidden  from  him  below,  while  the  heavenly  bodies 
are  equally  veiled  from  him  above;  but  rising  still  higher,  he 
again  emerges  to  brightness,  and  looks  down  upon  the  fleecy 
ocean  rolled  in  mountain  heaps  beneath  him,  as  the  climber  to 
a  lofty  peak  may  look  down  from  the  ever  pure  atmosphere 
around  it  on  the  inferior  region  of  clouds  and  storms. 

The  diminished  temperature  of  air  in  the  higher  regions  of 
the  atmosphere,  often  enables  the  natives  of  temperate  climates, 
when  led  by  circumstances  to  reside  in  tropical  countries, 
where  their  health  may  suffer  from  the  heat,  to  find  near  at 
hand,  on  some  mountain  height,  the  congenial  temperature 
of  their  early  homes.  The  author  once,  during  a  visit  to  the 
recently  inhabited  island  of  Penang,  in  the  strait  of  Malacca, 
examined  this  fact  with  pleasure  not  readily  forgotten.  The 
centre  of  the  island  is  occupied  by  a  lofty  mountain  ridge  thick- 

7 


50  HEAT. 

ly  wooded,  on  the  northern  summit  of  which  a  few  residences, 
visible  from  the  sea-shore  like  eagles'  nests  on  a  cliff,  had  just 
been  constructed.  Towards  these,  one  morning  at  sun-rise,  on 
an  active  little  horse  of  the  country,  and  along  a  tolerable  road, 
he  began  to  climb  from  the  hot  plain  below.  At  first  there 
were  around  him  purely  tropical  objects,  inspiring  tropical  feel- 
ings,— the  latter,  modified,  indeed,  by  the  reflection  that  his 
track  lay  through  a  forest,  into  which  until  lately  the  foot  of 
man  had  never  ventured,  and  where  the  trees,  nursed  through 
ages  to  their  greatest  growth,  and  the  stupendous  precipices, 
and  the  sublime  water-fall,  &c.  had  so  recently  been  exposed 
to  human  observation;  but,  as  he  gradually  ascended,  the  cha- 
racter of  the  vegetation  was  perceived  to  be  changing,  and  the 
air  was  becoming  so  light  and  cool  as  irresistibly  to  awaken  in 
him  thoughts  of  distant  England — nay,  almost  the  illusion  of 
his  being  there.  At  last,  however,  the  summit  being  reached, 
where  a  clear  space  opened  to  view  the  whole  country  around, 
the  attention  was  quickly  recalled  to  the  fervid  land  of  the  sun. 
The  elevation  is  so  great  that  at  first  the  eye  takes  account  of 
only  the  grander  features  of  the  scene,  and  such  nearly  as 
might  be  met  with  on  a  Grecian  or  Italian  shore:- — the  expanse 
of  sunny  water  in  that  beautiful  strait,  stretching  so  far  north 
and  south,  the  opposite  continental  shore  with  its  river  wind- 
ing seaward  across  the  plain,  the  town  and  the  roadstead  near 
it  crowded  with  ships,  which  appeared  only  as  specks  in  a 
wide-spread  map,  &c. ;  but,  on  closer  inspection,  and  particu- 
larly with  the  aid  of  the  telescope,  were  descried  the  rich 
groves  of  cocoa  nut  and  banana,  the  plantations  of  spice  and 
cotton  and  sugar  cane,  the  tawny  labourers,  the  bamboo  dweK 
lings,  the  fanciful  canoes  or  prows,  in  a  word,  every  object  be- 
speaking the  torrid  zone.  Such  then  is  the  scene,  which  even 
under  the  equator,  an  invalid  by  climbing  a  hill  may  place  un- 
der his  eye,  and  where  the  thermometer  near  him  stands  as  in 
an  English  month  of  May. 

The  interiors  of  the  islands  of  Jamaica  and  Hayti  have  many 
situations  of  great  extent,  which  combine,  as  above  described, 
the  advantages  of  tropical  situation  and  temperate  climate,  and 
English  labouring  colonists  might  well  inhabit  the  former. — 


CAPACITY  OP  BODIES.  51 

The  vast  plain  of  Mexico,  and  much  of  the  central  land  of 
South  America,  is  similarly  circumstanced;  and  it  is  not  un- 
common, where  the  ascent  to  the  gigantic  Andes  is  gradual, 
to  find  at  the  bottom  of  the  ridge  a  town,  whose  markets  are 
stored  only  with  the  productions  of  the  equator,  while  in  a 
town  higher  up  will  be  seen  only  what  belongs  to  the  tempe- 
rate skies  of  Europe; — climates  of  the  earth,  naturally  distant, 
thus  having  met,  as  it  were,  in  amicable  vicinity  on  the  same 
rising  plain. 

Second.  It  might  be  anticipated  that  a  dense  body,  or  one 
in  which  the  constituent  particles  may  be  supposed  to  fill  more 
completely  the  space  occupied  by  it  than  the  particles  do  in  a 
rarer  body,  would  have  smaller  capacity  for  heat,  in  proportion 
to  the  smaller  space  left  vacant  in  its  mass:  and  in  a  general 
comparison  of  the  capacities  of  equal  bulks  of  different  sub- 
stances, such  anticipation  is  partly  verified, — as  when  a  pint 
of  dense  mercury  is  found  to  have  only  about  half  the  capaci- 
ty which  a  pint  of  lighter  water  has.  The  accordance,  howe- 
ver, is  by  no  means  universal,  nor  at  all  in  proportion  to  the 
differences  of  density.  Water,  which  is  denser  than  oil,  and 
according  to  the  hypothesis  should  have  less  capacity,  yet  has 
in  the  same  volume  nearly  double  the  capacity;  and  mercury, 
which  being  nearly  fourteen  times  denser  than  water,  might  be 
expected  to  have  only  a  fourteenth  of  the  capacity,  has  really, 
for  equal  volumes,  a  half,  or,  as  formerly  stated,  for  equal 
weights,  a  thirtieth. 

Third.  We  are  at  last,  therefore^  compelled  to  admit,  that 
the  relation  between  various  substances  and  heat,  which  we 
call  capacity  for  heat,  depends  much  more  on  the  nature  of  the 
ultimate  atoms  of  the  substances  than  either  on  the  absolute 
bulk  or  comparative  density  of  the  masses.  Throwing  much 
light  on  this  subject,  it  has  been  ascertained  in  late  times,  that 
all  material  substances  are  composed  of  extremely  minute  un- 
changeable atoms,  and  of  which,  in  different  substances,  the 
comparative  weights  have  been  determined,  although  not  the 
absolute  weights;  that  is  to  say,  for  instance,  the  atom  of  gold 
is  known  to  weigh  four  times  as  much  as  the  atom  of  iron,  al- 
though we  do  not  know  ho  w many  thousands  or  millions  of  atoms 


52  HEAT. 

are  required  to  form  a  grain  of  either.  Now,  very  recent  re- 
searches seem  to  prove  that  for  each  ultimate  atom,  no  matter 
of  what  substance,  nearly  the  same  quantity  of  heat  is  required 
to  produce  in  a  mass  of  the  atoms  a  given  change  of  tempera- 
ture. Thus  an  ounce  of  iron  which  has  four  times  as  many 
atoms  as  an  ounce  of  gold,  has  four  times  the  capacity  for  heat. 
The  law  seems  to  hold  for  all  simple  substances;  and  for  com- 
pounds of  these,  there  seems  to  be  another  law  not  yet  well 
made  out. 

Instead  of  the  term  capacity  for  heat  used  in  the  preceding 
pages,  with  respect  to  particular  substances,  that  of  specific 
heat  has*  by  some  authors  been  preferred ;  but  as  the  latter  gives 
to  a  commencing  student  the  idea  rather  of  specific  kinds  of 
heat  than  of  specific  quantities,  the  term  capacity  has  been 
here  retained. 

"  Each  substance  in  nature,  for  a  given  change  of  tempe- 
rature, undergoes  expansion  in  a  degree  proper  to  itself, 
the  expansion  generally  increasing  more  rapidly  than  the 
temperature,  as  the  cohesion  of  the  particles  becomes 
weaker  from  increased  distance,  being  remarkably  great- 
er, therefore,  in  liquids  than  in  solids,  and  in  airs  than 
in  liquids;  the  rate  being  quickened,  moreover,  near  the 
points  of  change.  (See  the  Analysis,  page  13.) 

The  following  table,  containing  the  names  of  some  common 
substances,  solid,  liquid,  and  aeriform,  shows,  by  the  figures 
following  each  name,  how  much  the  substance  increases  in 
bulk,  by  having  its  temperature  raised  from  that  of  freezing  to 
that  of  boiling  water.  A  lump  of  glass,  for  instance,  would 
gain  in  the  proportion  of  one  cubic  inch  for  every  416  cubic 
inches  contained  in  it;  while  a  mass  of  water  would  gain  one 
inch  or  part  in  twenty-three;  dilating  thus  for  the  same  range 
of  temperature  nine  times  more  than  the  glass. 

Solids. 

Glass  gains  one  part  in     -  -  416 

Deal  -  416 

•  Platinum 389 


EXPANSION  OF  BODIES.  53 

Steel       -                            ...  283 

Cast  iron  -  273 

Iron  .  271 

Gold  .  221 

Copper    -  -  194 

Brass       -  -  177 

Silver     -  -        -  175 

Tin                                    -  .170 

Lead       -  -  117 

Liquids. 

Mercury  gains  one  part  in  -  55 

Water     -  -  23 

Oil  of  turpentine     -  14 

Fixed  oils  -  12 

Alcohol  9 


Common  air,  i 
All  gases  and  >  gain  one  part 
vapours       J 


Airs. 

in 


We  have  to  warn  readers,  here,  not  to  confound  the  increase 
hy  heat  of  the  general  bulk  of  a  solid  body  with  the  increase 
of  its  length.  The  latter  is  only  one-third  as  great  as  the  for- 
mer. This  will  be  understood  by  considering  that  the  increase 
of  bulk  is  divided  between  the  breadth  and  depth  (or  thick- 
ness) in  common  with  the  length.  If  the  substance  of  a  me- 
tallic square  rod  or  wire,  for  instance,  be  dilated  by  heat,  the 
hundredth  part  of  its  bulk,  it  does  not  gain  all  that  hundredth 
at  its  end,  becoming  perhaps  101  inches  long  instead  of  100; 
but  every  part  becoming  deeper  and  broader  in  the  same  pro- 
portion as  it  becomes  thicker,  (we  may  suppose  it  divided 
into  a  row  of  equal  little  cubes,)  the  rod  gains  in  length 
only  the  third  part  of  an  inch.  A  fluid  enclosed  in  a  tube  un- 
changeable by  heat  (if  such  tube  there  were)  would  show  its 
whole  dilatation  in  an  increase  of  length,  because  there  could 
be  no  swelling  laterally,  and  its  extremity  would,  therefore, 


54  HEAT. 

have  a  triple  extent  of  motion  from  any  variation  of  tempera- 
ture. A  degree  of  this  consequence  is  obtained  in  our  com- 
mon thermometers,  because  the  containing  glass,  although  di- 
latable by  heat,  is  so  much  less  dilatable  than  the  fluid  within. 
As  regards  solids,  we  have  to  inquire  so  much  more  frequent- 
ly respecting  the  dilatation  in  length,  breadth,  &c.;  that  is  to 
say,  the  linear  dilatation  in  some  direction,  than  the  increase 
of  general  bulk,  that  tables  are  frequently  made  stating  only 
the  linear  dilatation.  It  may  be  found  at  once  from  the  above 
table,  by  recollecting  that  it  is  one-third  of  the  increase  of 
bulk: — thus,  if  glass,  in  passing  from  the  freezing  to  boiling 
heat  of  water,  dilate  one  part  in  416  of  its  bulk,  it  will  dilate 
only  the  third  of  a  part  in  length,  or  a  whole  part  in  an  extent 
of  three  times  416  or  1,248. 

The  expansion  by  heat  of  solids  has  been  ascertained  by 
bringing  microscopic  instruments  to  bear  on  rods  of  the  vari- 
ous substances  heated  to  various  degrees,  in  troughs  of  oil  or 
water.  The  expansion  of  fluids,  again,  is  found  by  filling  a 
glass  vessel  with  a  known  weight  of  any  fluid,  and  then  ascer- 
taining how  much  is  made  to  run  over  or  escape  by  a  given  in- 
crease of  heat.  This  quantity,  added  to  what  is  required  to  fill 
the  increased  dimensions  of  the  heated  glass  vessel,  (which 
from  the  ascertained  expansion  of  glass  is  known,)  forms  the 
whole  of  the  increase.  It  might  be  ascertained,  also,  by  put- 
ting different  liquids  successively  into  the  thermometer  tube, 
and  marking  their  comparative  dilatations  from  changes  of  tem- 
perature examined  by  another  thermometer. 

The  general  and  comparative  expansion  of  solids  by  heat  is  ex- 
emplified in  the  following  cases: 

A  cannon  ball,  when  heated,  cannot  be  made  to  enter  an 
opening,  through  which,  when  cold,  it  passes  readily. 

A  glass  stopper  sticking  fast  in  the  neck  of  a  bottle  often 
may  be  released  by  surrounding  the  neck  with  a  cloth  taken 
out  of  warm  water — or  by  immersing,  the  bottle  in  the  water 
up  to  the  neck:  the  binding  ring  is  thus  heated  and  expanded 
sooner  than  the  stopper,  and  so  becomes  slack  or  loose  upon  it 

Pipes  for  conveying  hot  water,  steam,  hot  air,  &c.;  if  of  con- 


EXPANSION  OP  SOLIDS.  55 

siderable  length,  must  have  joinings  that  allow  a  degree  of 
shortening  and  lengthening,  otherwise  a  change  of  temperature 
may  destroy  them.  An  incompetent  person  undertook  to  warm 
a  large  manufactory  hy  steam  from  one  boiler.  He  laid  a  rigid 
main  pipe  along  a  passage,  and  opened  lateral  branches  through 
holes  into  the  several  apartments,  but  on  his  first  admitting  the 
steam,  the  expansion  of  the  main  pipe  tore  it  away  from  all 
its  branches. 

In  an  iron  railing,  a  gate  which  during  a  cold  day  may  be 
loose,  and  easily  shut  or  opened,  in  a  warm  day  may  stick, 
owing  to  there  being  greater  expansion  of  it  and  of  the  neigh- 
bouring railing,  than  of  the  earth  on  which  they  are  placed. 
Thus,  also,  the  centre  of  the  arch  of  an  iron  bridge  is  higher 
in  warm  than  in  cold  wreather;  while,  on  the  contrary,  in  a  sus- 
pension or  chain  bridge,  the  centre  is  lowered. 

The  iron  pillars  now  so  much  used  to  support  the  front  walls 
of  houses  of  which  the  ground  stories  serve  as  shops,  with  spa- 
cious windows,  in  warm  weather  really  lift  up  the  wall  which 
rests  upon  them,  and  in  cold  weather  allow  it  again  to  sink  or 
subside — in  a  degree  considerably  greater  than  if  the  wall  were 
brick  from  top  to  bottom. 

In  some  situations,  (as  lately  was  seen  in  the  beautiful  steeple 
of  Bow  church,  in  London,)  where  the  stones  of  a  building  are 
held  together  by  clamps  or  bars  of  iron,  with  their  ends  bent 
into  them,  the  expansion  in  summer  of  these  clamps  will  force 
the  stones  apart  sufficiently  for  dust  or  sandy  particles  to  lodge 
between  them:  and  then,  on  the  return  of  winter,  the  stones 
not  being  at  liberty  to  close  as  before,  will  cause  the  ends  of  the 
shortened  clamps  to  be  drawn  out,  and  the  effect  increasing 
with  each  revolving  year,  the  structure  will  at  last  be  loosened, 
and  may  fall. 

The  pitch  of  a  piano-forte  or  harp  is  lowered  in  a  warm  day 
or  in  a  warm  room,  owing  to  the  expansions  of  the  strings  be- 
ing greater  than  of  the  wooden  frame-work;  and  in  cold  the 
reverse  will  happen.  A  harp  or  piano,  which  is  well  tuned  in 
a  morning  drawing-room,  cannot  be  perfectly  in  tune  when  the 
crowded  evening  party  has  heated  the  room. 


56  HEAT. 

Bell-wires  too  slack  in  summer,  may  be  of  the  proper  length 
in  winter. 

One  admirable  contrivance  for  keeping  the  pendulum  of  a 
clock  always  of  the  same  length,  by  making  the  greater  expan- 
sion by  heat  of  a  middle  bar  of  brass  counteract  the  smaller  ex- 
pansion of  two  side  rods  of  steel,  was  explained  in  vol.  i.,  un- 
der the  head  of  6  Pendulum,'  as  was  also  the  construction  of 
a  balance  wheel  having  a  corresponding  property.  A  differ- 
ence of  a  hundredth  of  an  inch  in  the  length  of  a  common  pen- 
dulum, causes  a  clock  to  err  ten  seconds  in  twenty-four  hours, 
and  a  rise  or  fall  of  25°  of  Fahrenheit's  thermometer  produces 
this  difference.  Another  kind  of  compensation  pendulum,  dis- 
tinguished by  the  name  of  its  inventor,  Graham,  is  obtained 
by  substituting  for  the  solid  bob  or  ball  at  the  bottom,  a  glass 
vessel  containing  mercury.  The  mercury  on  expanding  by 
heat  has  its  centre  of  gravity  raised  just  enough  to  compensate 
for  the  lengthening  of  the  rod  of  the  pendulum. 

Crystals  do  not  expand  quite  equally  in  breadth  and  in 
length,  and  the  expansion  of  one  part  may  even  cause  a  con- 
traction of  a  part  not  yet  warmed.  The  same  is  true  of  fibrous 
substances,  as  wood,  which  expands  and  contracts  more  in 
breadth  than  in  length.  This  is  proved  by  the  leaking  in  cold 
weather  of  a  ship's  deck,  which  in  warm  weather  is  tight:  an 
occurrence  which  the  author  once,  in  rounding  the  Cape  of 
Good  Hope,  had  to  regret  as  the  cause  of  destruction  to  some 
valuable  specimens  of  natural  history  which  he  had  collected 
among  the  eastern  islands. 

Other  interesting  examples  of  expansion  and  contraction  in 
solids  might  be  mentioned,  but  the  above,  in  addition  to  what 
were  given  in  vol.  i.  under  the  head  of  '  Repulsion,9  may  suf- 
fice. Bodies  expanded  by  heat,  unless  when  their  intimate 
composition  is  changed  by  it,  regain  exactly  their  former  di- 
mensions on  being  cooled. 

As  is  seen  in  the  preceding  table,  the  expansion  of  liquids  is 
much  greater  than  of  solids. 

A  cask  quite  filled  with  liquid  in  winter,  must  force  its  plug 
in  summer,  or  must  burst:  and  a  vessel  which  has  been  filled 


EXPANSION  OF  AIRS.  57 

to  the  lip  with  warm  liquid,  will  not  be  full  when  the  liquid 
has  cooled.  Hence,  some  cunning  dealers  in  liquids  try  to 
make  their  purchases  in  very  cold  weather,  and  their  sales  in 
warm  weather. 

There  exists  a  most  extraordinary  exception,  already  men- 
tioned, to  the  law  of  expansion  by  heat,  and  contraction  by  cold, 
producing  unspeakable  benefits  in  nature,  viz.  in  the  case  of  wa- 
ter. Water  contracts,  according  to  the  law,  only  down  to  the 
temperature  of  40°,  while,  from  that  to  32°,  which  is  its  freezing 
point,  it  again  dilates.  A  very  curious  consequence  of  this  pe- 
culiarity is  exhibited  in  the  wells  of  the  glaciers  of  Switzer- 
land and  elsewhere,  namely,  that  when  once  a  pool  or  shallow 
well  on  the  ice  commences,  it  goes  on  quickly  deepening  itself, 
until  it  penetrates  to  the  earth  beneath.  Supposing  the  suv- 
face  of  the  water  originally  to  have  nearly  the  temperature  of 
the  melting  ice,  or  32°,  but  to  be  afterwards  heated  by  the  air 
and  sun,  instead  of  the  water  being  thereby  dilated  or  render- 
ed specifically  lighter,  and  detained  at  the  surface,  it  becomes 
heavier  the  more  nearly  it  is  heated  to  40°,  and  therefore  sinks 
down  to  the  bottom  of  the  pit  or  well;  but  there,  by  dissolving 
some  of  the  ice,  and  being  consequently  cooled,  it  is  again  ren- 
dered lighter,  and  rises  to  be  heated  as  before,  again  to  de- 
scend; and  this  circulation  and  digging  cannot  cease  until  the 
water  has  bored  its  way  quite  through. 

Airs  are  expanded  by  heat  still  more  than  liquids. 

The  expansion  of  aeriform  bodies  by  heat  produces  many 
important  effects  in  nature.  Some  of  these  have  already  been 
considered  in  the  preceding  parts  of  this  work,  as,  the  rising 
of  heated  air  in  the  atmosphere,  causing  the  winds  all  over  the 
earth;  the  same  in  our  fires  and  chimneys  supporting  combus- 
tion, and  ventilating  and  purifying  our  houses;  the  same,  again, 
from  around  animal  bodies,  removing  the  poisonous  or  conta- 
minated air  that  issues  from  the  lungs,  and  ensuring  a  constant 
supply  of  fresh  air  for  the  support  of  life,  &c. 

It  is  remarkable,  with  respect  to  aeriform  bodies,  that  they 
are  all  equally  dilated  by  the  same  change  of  temperature,  re- 
ceiving ao.  increase  of  about  a  third  part  of  their  bulk 

S 


5S  HEAT. 

parts  in  100)  on  being  heated  from  the  freezing  to  the  boiling 
point  of  water,  viz.  ISO0,  and  their  bulk  being,  therefore^  dou- 
bled from  the  same  standard  point  by  about  500°.  This  gene- 
ral truth  holds,  not  only  with  respect  to  the  more  permanent 
airs  or  gases,  but  also  with  respect  to  all  steams  or  vapours  in 
the  dry  state,  that  is,  when  not  in  contact  with  the  liquid  pro- 
ducing them.  The  probable  reason  of  this  uniformity  is,  that 
cohesive  attraction,  which  varies  so  much  in  different  solids, 
modifying  the  effects  of  heat  upon  them,  in  aeriform  fluids  does 
not  exist  at  all. 

The  extent  of  this  dilatation  for  airs  is  so  much  greater  than 
for  liquids  or  solids,  that  it  forces  itself  much  more  strikingly 
upon  the  common  attention.  Thus  a  bladder  containing  con- 
siderably less  than  its  fill  of  air,  becomes  tense  immediately  on 
being  held  to  the  fire.  The  air  in  a  balloon  just  escaping  from 
a  cloud  has  been  so  suddenly  expanded  by  the  direct  rays  of 
the  sun,  as  to  have  injured  the  texture  of  the  balloon;  and 
probably  some  of  the  fatal  accidents  among  aeronauts  have 
thus  arisen.  Burning  fuel  conveyed  into  a  vessel  or  case  which 
can  be  suddenly  and  strongly  closed,  will  produce  an  expansion 
of  the  air,  confined  with  it,  capable  of  bursting  any  ordinary 
material — in  short,  will  produce  an  explosion. 

Now,  if  not  before,  at  any  rate,  soon  after  steam  engines 
began  to  be  used,  and  had  so  strikingly  shown  to  what  impor- 
tant purposes  the  force  of  an  expanding  aeriform  fluid  might  be 
applied,  the  thought  would  naturally  occur,  that  the  force  of  com- 
mon air,  dilating  by  heat,  might  also  be  rendered  useful.  Accord- 
ingly, a  variety  of  air-expansion  engines  has  been  proposed, 
but  as  yet  no  one  has  been  reduced  to  profitable  practice.  Had 
the  truth  been  generally  known,  which  very  recent  investiga- 
tions have  proved,  that  any  given  quantity  of  heat,  when  used 
to  dilate  air,  produces  about  four  times  the  quantity  of  expan- 
sive power  that  it  does  when  used  to  form  steam,  the  attempts 
to  bring  such  an  application  of  heat  under  control  would  pro- 
bably have  been  much  more  numerous,  and  possibly,  by  this 
time,  in  a  degree  successful.  The  subject  is  so  interesting  that 
we  shall  subjoin  a  few  remarks  upon  it. 

To  produce  a  cubic  foot  of  common  steam,  from  water  ori- 


EXPANSION  OF  ATRS.  59 

finally  cold,  about  1,150  degrees  of  heat  are  required,  as  will 
be  explained  a  few  pages  hence.  The  same  quantity  of  heat 
would  double  the  volume  of  about  five  cubic  feet  of  atmosphe- 
ric air, — as  is  known  from  the  comparative  capacities  for  heat 
of  the  two  substances,  and  the  rate  of  dilatation  of  air  when 
heated.  Now  the  value  for  work  of  the  foot  of  steam  passing 
from  the  boiler  into  a  working  cylinder  would  be,  to  press  up 
the  piston  of  the  steam  engine  through  a  foot,  as  from  c  d  to  a  b, 
with  a  force  all  the  way  of  15  Ibs.  per  inch  of  the  piston  sur- 
face; while  the  working  value  of  the  five  feet  of  air,  in  di- 
lating to  double  bulk,  would  be  to  lift  the  piston  five  times  as  far 
as  the  steam,  vis.  from  g  h  to  e  f,  but 
with  a  force  gradually  diminishing  as 
the  expansion  went  on,  from  15  Ibs. 
per  inch  at  the  beginning  until  the  air 
had  dilated  to  its  destined  volume,  when 
the  force  would  altogether  cease:  its 
whole  effect,  therefore,  would  be  five 
feet  impulsion  of  the  piston,  with  a  pres- 
sure, the  average  between  15  Ibs.  and 
•  nothing,  viz.  7%  Ibs.  per  inch;  and  the 
friction  in  the  two  cases  and  the  varying  intensity  of  the  latter 
pressure  being  neglected,  the  force  of  the  air  would  be  2k  times 
as  great  as  that  of  the  steam.  But  it  is  farther  to  be  considered, 
that  only  about  half  the  heat  of  a  fire  is  applied  to  use  in  the 
steam  engine,  viz.  that  part  which  enters  the  boiler,  while  the 
remainder  p;)v  es  up  the  chimney;  and  in  an  air  engine  proba- 
bly the  whole  might  be  applied.  In  an  air  engine,  moreover, 
there  might  be  great  increase  of  power  from  the  combustion,  or 
semi-explosion  of  the  inflammable  gas  evolved  from  the  fuel. — 
As  it  was  easy,  long  before  the  steam  engine  was  contrived,  to 
determine  the  expansive  force  of  steam,  and  to  compare  it  with 
any  other  force;  by  proceeding  in  a  similar  way  with  respect 
to  heated  air,  we  may  estimate  its  expansive  power  to  be  four 
times  greater  than  that  of  steam  from  an  equal  quantity  of  fuel, 
and  when  used  at  a  common  or  low  pressure.  We  see  from 
this  of  what  importance  the  discovery  would  be,  of  a  means 
enabling  us  etiectually  to  apply  the  force  of  expanding  air. 


HEAT. 


If  we  suppose  a  fire,  «,  to  be  placet 
on  a  grate  near  the  bottom  of  a  close 
cylinder,  d  a,  and  the  cylinder  to  be 
full  of  fresh  air  recently  admitted,  and 
if  we  then  suppose  the  loose  piston,  g  d, 
to  be  pulled  upwards,  it  is  evident  that 
all  the  air  in  the  cylinder  above  d  will  be 
made  to  pass  by  the  tube  e  through  the 
fire,  and  will  receive  an  increased  elas- 
ticity tending  to  the  expansion  or  increase  of  volume,  which  the 
fire  is  capable  of  giving  it.  If  there  were  only  the  single  close 
vessel  d  a,  the  expansion  might  be  so  strong  as  to  burst  it;  but 
if  another  vessel,  b  c,  of  equal  size  were  provided,  communica- 
ting with  the  first  through  the  passage  6,  and  containing  a  close- 
fitting  piston,  cf9  like  that  of  a  steam  engine,  the  expansion  of 
the  air  would  act  to  lift  the  said  piston,  and  by  means  of  it 
might  work  water  pumps,  or  do  any  other  service  which  a  steam 
engine  can  perform.  At  the  end  of  the  lifting  stroke  of  the  pis- 
ton,/ c,  it  might  be  made  to  open  an  escape  valve  for  the  hot 
air,  placed  in  any  convenient  part  of  the  apparatus,  and  to  cause 
the  descent  of  the  blowing  piston,  d,  to  expel  this,  while  a  new 
supply  of  air  would  enter  by  another  valve  into  the  cylinder 
above  d.  The  engine  would  then  be  ready  to  repeat  its  stroke 
as  before,  and  the  working  would  be  continued  as  in  a  steam 
engine. 

The  preceding  simple  conception  of  an  air  engine  occurred 
to  the  author's  thoughts  while  considering  the  application  of  a 
condensed  air  furnace  to  some  chemical  purposes.  It  appeared 
to  him  that  the  chief  difficulties  to  be  surmounted  in  applying 
any  such  engine  to  use  would  be,  to  prevent  the  very  heated  air 
and  dust  from  injuring  the  valves  and  other  working  parts  of 
the  engine,  and  to  obviate  the  inconvenience  of  the  inequality 
of  power  at  different  parts  of  the  stroke.  Various  expedients 
occurred  to  him.  The  overheating  might  be  prevented  by  sur- 
rounding the  cylinder,  &c.  with  water;  and  both  cylinder  and  pis- 
ton would  suffer  less  from  dust  if,  instead  of  the  common  piston, 
c,  represented  above,  a  great  hollow  plunger,  «,  were  used  (such 
as  is  here  represented,  and  is  now  common  in  water  pumps  for 


EXPANSION  OP  AIRS. 


tfl 


£t> 


mines,)  embraced  by  an  air-tight  neck  or  col- 
lar at  b  c,  which  neck  would  be  the  only  part 
J-l — r  of  the  cylinder  requiring  to  be  made  with 
nicety.  But  a  more  complete  security  would 
be  obtained  by  interposing  water  between  the 
hot  air  and  the  piston,  as  represented  in  this 
other  sketch,  where  the  working  cylinder,  d, 
has  a  water  vessel,  b,  connected  with  it,  and 
the  heated  air  is  admitted  to  press  upon  a 
float,  on  the  water-surface,  to  lift  the  work- 
ing piston  d  e.  This 
construction,  too,  if  de- 
sired, would  allow  the 
fire  chamber,  a,  to  be 
made  larger  than  the 
cylinder,  and  to  be  kept 
constantly  filled  with 
highly  expansive  air, 
each  discharge  of  which 
into  the  space  b  would  be  replaced  by  cold  air  either  from  the 
space  above  d,  driven  in  through  a  tube  as  the  piston  ascended, 
or  from  a  distinct  blowing  cylinder  worked  by  the  beam.  And 
if  it  were  wished  to  apply  the  same  principle  to  an  engine  work- 
ing with  double  strokes,  that  is,  forcing  the  piston  alternately 
up  and  down,  as  in  the  double  stroke  steam  engine,  the  object 
might  be  attained,  by  having  a  second  water  vessel,  /,  commu- 
nicating with  the  part  of  the  working  cylinder  above  the  piston 
d;  and  the  air  would  pass  alternately  to  the  one  or  the  other  ves- 
sel b  or  f9  by  the  operation  of  the  cock  c,  as  steam  passes  in  a 
steam  engine:  the  supply  of  fresh  air  to  the  chamber  a  would 
be  given  by  a  blowing  cylinder  worked  through  a  connexion 
with  the  engine,  as  the  air  pump  of  a  steam  engine  is  worked. 
The  sketch  of  an  air  engine,  as  here  given,  was  included  in 
the  specification  of  a  patent  for  another  object  engaged  in  some 
years  ago  by  a  friend  of  the  author's;  but  he  being  almost  im- 
mediately called  to  other  business,  and  the  author's  professional 
engagements  precluding  his  attention  to  the  subject,  it  was  not 
prosecuted.  Jn  the  specification,  drawn  up  by  an  engineer  in 


HEAT. 


town,  some  minor  adaptations  were  described.  One  experiment 
has  lately  been  made  by  a  Swedish  engineer  with  the  simple 
form  of  dry  apparatus  described  at  page  59,  for  the  purpose  of 
ascertaining  its  power,  and  the  effect  was  found  to  be  several 
fold  greater  than  of  steam  from  the  same  quantity  of  fuel;  but 
the  apparatus  was  rude,  and  only  calculated  to  prove  in  a  short 
trial,  the  existence  of  the  power,  but  not  the  fitness  of  the  ma- 
chine to  endure  long  uninjured,  and  to  be  rendered  easily  obe- 
dient to  control:  a  complete  experiment,  therefore,  remains  still 
to  be  made.  Could  an  obedient  and  durable  engine  be  con- 
trived, at  all  approaching  in  simplicity  to  the  plan  given  above, 
its  advantages  over  the  steam  engine  would  be  very  considerable. 
First,  its  original  cost  would  be  much  less,  by  reason  of  its 
small  comparative  size,  its  simplicity,  and  the  little  nicety  of 
workmanship  required.  Secondly,  it  would  occupy  much  less 
room,  and  would  be  very  light;  hence,  its  peculiar  fitness  for 
purposes  of  propelling  ships  and  wheel  carriages.  Thirdly,  the 
quantity  of  fuel  required,  being  so  much  less,  would  not  load 
the  ship  or  carriage,  leaving  little  room, 
as  happens  in  steam-boats,  for  any  thing 
else.  Fourthly,  the  expense  of  fuel  and 
repairing  would  be  little.  Fifthly,  the 
engine  could  be  set  to  work  in  a  few 
minutes,  where  a  steam  engine  might 
require  hours.  Sixthly,  little  or  no  wa- 
ter would  be  required  for  it. 

Another  modification  of  air  engine, 
called  a  gas  vacuum  engine,  has  late- 
ly been  proposed,  and  many  expensive 
trials  have  been  made  of  it;  but  it  is  in 
its  nature  a  most  wasteful  machine, 
evidently  throwing  away  at  least  nine- 
tenths  of  the  power  which  its  princi- 
ple generates.  It  was  of  this  nature  in 
an  experiment  which  the  author  wit- 
nessed. A  little  of  the  common  coal 
gas  was  admitted  by  the  cock  b  at  the 
bottom  of  the  cylinder  a,  and  was  there 


EXPANSION  OP  AIRS.  G3 

inflamed,  the  lid,  c,  being  at  the  time  raised.  The  combustion 
rarified  the  lower  stratum  of  air,  so  that  the  air  above  was  ex- 
pelled, and  about  one-fifth  of  the  original  contents  of  the  cylin- 
der was  made  to  occupy  the  whole.  The  lid  was  shut  down  as 
nearly  as  could  be  judged  at  the  moment  of  greatest  expansion, 
so  that  when  the  small  portion  of  air  and  vapour  remaining  was 
again  cooled,  the  interior  of  the  cylinder  approached  nearly  to 
the  state  of  vacuum.  It,  in  fact,  retained  only  a  fifth  of  the 
air.  A  communication  being  then  opened  by  the  tube  e,  from 
the  vacuous  cylinder  to  a  water  reservoir  ten  feet  below,  the 
water  was  driven  up  by  the  atmospheric  pressure,  and  filled 
more  than  half  of  the  cylinder.  The  water  so  raised  was  then 
made  to  turn  a  common  water  wheel,  and  so  to  do  work.  A 
larger  quantity  of  water,  however,  could  be  raised  to  the  same 
height  at  less  expense  by  a  steam  engine.  The  proposer  also 
hoped,  that  he  would  be  able  to  make  the  atmosphere  pressing 
into  his  imperfect  vacuum,  act  directly  upon  a  piston  as  steam 
does,  and  with  power  cheaper  than  that  of  steam;  but  in  this 
anticipation,  too,  he  was  completely  in  error.  To  produce  his 
imperfect  vacuum,  cost  him  very  nearly  at  the  same  rate  as  it 
costs  to  produce  the  perfect  vacuum  in  a  steam  engine,  and  his 
vacuum  for  equal  bulks  was  worth,  as  a  working  power,  only 
about  one-fourth  as  much  as  the  steam  vacuum.  This  may  be 
understood  by  considering  that  in  a  perfect  .vacuum  a  piston 
rises  all  the  way  with  the  same  force,  which,  if  common  steam 
be  used,  is  15  Ibs.  per  inch  (the  piston  may  be  supposed  to  rise 
from  c  d  to  a  b,)  but  if  the  vacuum  were  only 
three-fourths  towards  being  perfect,  the  pres- 
sure on  the  piston  would  be  only  three-fourths 
of  15  Ibs.  at  the  commencement  of  the  stroke, 
would  then  rapidly  diminish,  and  would  have 
ceased  altogether  when  the  piston  had  made 
three-quarters  of  its  journey,  or  to  /  The 
force  in  the  first  case  would  be  represented  by  the  whole  line 
c  d  and  the  square  space  cdba,  and  in  the  second  by  the  short- 
ening lines  and  the  triangular  space  cef. 

On  considering  the  foregoing  diagrams  we  may  perceive  that 
in  the  vacuum  engine,  by  far  the  greater  part  of  the  force  pro- 


64  HEAT. 

duced  by  the  combustion  of  the  gas  is  absolutely  wasted,  or  put 
to  no  use,  viz.  the  whole  expansive  force  during  the  sudden 
combustion  or  explosion.  It  is  evident  that  if  a  tenth  part  of 
the  aeriform  contents  of  a  cylinder  acquire  elasticity  enough, — 
and  a  fourteenth  part  in  a  nice  experiment  does  so — to  be  able 
afterwards  to  occupy  the  whole  cylinder,  it  must  begin  its  ex- 
pansion with  the  force  of  a  tenfold  atmospheric  condensation, 
that  is,  a  pressure  of  150  Ibs  on  the  square  inch  of  a  piston 
withstanding  it,  which  pressure  will  then  gradually  diminish  as 
the  piston  rises,  but  will  amount  to  an  average  of  five  times  the 
atmospheric  pressure,  or  75  Ibs.  per  inch  all  the  way;  being, 
therefore,  quadruple  or  more,  that  of  steam  against  a  perfect  va- 
cuum; and,  therefore,  again,  by  our  former  calculation,  at  least 
twelve  times  greater  than  the  force  obtained  from  the  imperfect 
vacuum  of  the  engine  under  consideration. 

It  is  a  question  which  the  author  thinks  will  one  day  be  an* 
swered  in  the  affirmative,  whether  nearly  the  whole  force  of  ex- 
ploding gas  may  not  be  converted  into  a  calmly  working  power, 
producing  from  a  given  expenditure,  ten  times  or  more  the  ef- 
fect obtained  in  the  vacuum  engine  described  above,  and  there- 
fore more  than  from  a  steam  engine  incurring  the  same  expense. 
There  are  probably  various  ways  in  which  the  object  may  be 
attained.  The  following  sketch  is  offered  merely  to  give  the 
reader  an  idea  of  a  machine  for  such  a  purpose. 

Suppose  b  to  be  a  very  heavy  close-fitting 
piston  sliding  in  the  cylinder  around  it,  and 
suppose  the  space  d  open  to  the  cylinder,  to  be 
filled  with  atmospheric  air  of  double  or  great- 
er density;  then  if  a  mixture  of  explosive 
gases,  admitted  by  a  cock  to  the  chamber  a 
(formed  between  the  piston  and  end  of  the 
cylinder)  be  inflamed,  the  heavy  piston  will 


•TT-  be  shot  forward,  like  a  cannon  ball,  against 

"&  the  condensed  air  in  d;  and  owing  to  the 

momentum  acquired  in  the  first  instance,  it  will  advance  much 
beyond  the  point  where  the  exploded  gas  and  air  in  d  would 
balance  each  other  at  rest:  the  quantity  of  gases  admitted  would 
be  just  such  as  to  carry  it  to  the  end  of  the  cylinder.  The  pis- 


EXPANSION  OF  AIRS.  65 

ton  rod,  e,  would  then  by  a  catch,  or  ratchet,  he  connected  with 
the  work  to  be  done,  and  after  the  condensation  of  the  exploded 
gases  in  the  cylinder,  would  be  pressed  back  again,  with  the 
double  or  greater  than  atmospheric  force  in  d,  as  if  urged  by 
high  pressure  steam.  The  first  figure  at  page  61  represents  a 
form  of  cylinder  which  might  also  answer  for  this  purpose,  the 
heavy  plunger  being  thrown  up,  to  work  by  its  weight  in  de- 
scending. 

It  is  to  be  remarked,  that  the  first  modification  of  air  engine 
described  at  page  72,  is  partly  an  explosive  engine,  such  as  con-* 
templated  above;  for,  the  gas  separated  from  the  coal  during  the 
moment  of  slackened  combustion,  while  the  lately  used  air  is 
passing  out,  becomes  an  explosive  accumulation  for  the  fresh 
air  about  to  enter.  The  trial  alluded  to  above  proved  this  to  be 
the  fact. 

"  The  expansion  of  bodies  by  heat  increases  more  rapidlyf 
than  the  temperature,  and  particularly  near  the  melting 
and  boiling  points;  that  is,  their  points  of  changing  into 
liquid  or  air,  being,  however,  exactly  proportioned  to  the 
temperature  after  the  change  into  air."  (.See  the  Analysis, 
page  13.) 

If  a  given  quantity  of  heat,  of  that,  for  instance,  contained  in? 
some  measure  of  boiling  water  or  of  common  steam,  be  added 
to  a  mass  of  cool  water,  it  will  produce  in  this  a  certain  incre^ 
ment  of  bulk;  and  if  other  equal  quantities  of  heat  be  afterwards 
successively  added,  under  the  nice  management  which  such  art 
experiment  requires,  each  new  addition  will  produce  a  greater 
increment  of  bulk  than  the  preceding,  particularly  when  the? 
water  approaches  to  boiling;  but  after  the  Water  is  converted 
into  steam,  any  farther  increase  of  bulk  will  be  exactly  propor- 
tioned to  the  increase  of  temperature.  The  same  truths  may 
be  proved  by  the  converse  experiment  of  abstracting  successive- 
ly equal  quantities  of  heat  from  steam  or  water  (as  by  making 
it  melt  equal  quantities  of  ice,)  and  noting  the  rate  of  contrac- 
tion. What  is  thus  true  of  water  in  relation  to  heat,  is  truer 
also-  of  bodies  generally,  each,  however,  having  a  rate  of  ex-* 
pansion,  and  temperatures  for  melting  and  boiling  proper  to  it* 


66  HEAT. 

self.  The  quickened  rate  of  expansion  in  solids  and  liquids 
might  have  been  anticipated  from  reflecting,  that  each  successive 
quantity  of  heat  add,ed  to  a  mass,  meets  with  less  resistance  to 
its  expanding  power  than  the  preceding,  owing  to  the  diminish- 
ing force  of  cohesion  of  the  partiqles  as  the  mass  expands:  while 
in  an  air  or  gas,  again,  as  cohesion  has  altogether  ceased,  each 
addition  of  heat  is  at  liberty  to  produce  its  full  and  equal  effect. 
If  the  capacity  of  substances  for  heat  did  not  increase  with  their 
bulk,  the  terms  "  increase  of  heat"  and  "  increase  of  tempera- 
ture" would  have  the  same  meaning,  and  this  subject  would  be 
more  simple. 

The  reflection  will  naturally  occur  here,  that  as  in  the  com- 
mon thermometer  the  mercury  must  rise  or  expand  more  for  a 
given  quantity  of  heat  added  at  a  high  temperature  than  at  a  low 
temperature,  the  scale  should  be  divided  to  correspond  with  the 
inequality.  Now  this  reasoning  is  good,  but  the  difficulty  of 
complying  with  it  in  practice  is  such,  that  the  inconvenience  of 
the  slight  error  arising  from  an  equal  division  is  submitted  to. 
An  air  thermometer  with  equal  divisions  is  very  correct,  but 
from  wanting  many  of  the  advantages  of  the  mercurial  thermo- 
meter, is  little  employed;  and  fortunately  it  happens,  that  in 
the  mercurial  thermometer  there  is  such  a  counterbalancing  re- 
lation between  the  expansion  of  the  mercury  and  of  the  contain- 
ing glass,  as  to  render  the  error  alluded  to,  at  least  for  any  mid- 
dle range  of  temperature,  very  trifling.  The  subject  of  unequal 
thermometric  dilatation  in  the  same  liquid,  and  of  differences 
in  different  liquids,  depending  on  the  proximity  to  their  boiling 
points,  &c.,  is  well  illustrated  by  Du  Luc's  experiment  of  filling 
different  thermometer  glasses  with  different  liquids,  and  noting 
their  comparative  indications  when  heated  through  the  same 
range  of  temperature.  He  marked  on  each  the  points  at  which 
the  liquid  stood  when  the  glass  was  placed  in  freezing  and  in 
boiling  water,  and  then  divided  the  intervening  spaces  into 
eighty  parts  or  degrees.  The  discordance  of  their  dilatations 
is  here  detailed. 

Mercury.                 Olive  Oil.                          Alcohol.  Water. 

0  -     -     -     -     0      -----      0 0 

10  -     -     -     -     9.5  -----  7,9     -----     0.2 


LATENT  HEAT. 


Mercury. 

Olive  Oil. 

Alcohol. 

Water. 

20  -     - 

-     -  19.3  - 

-     -     -     -  16.5  - 

-     ...     4.1 

30  -     - 

-     -  29  3  - 

-  2*5  fi   - 

Ho 

40  -     - 

-     -  39.2  - 

-     -     -     -  35.1   - 

-     -     -     -  20.5 

50  -     - 

-     -  49.2  - 

-     -     -     -  45.3  - 

....  32 

60  -     - 

-     -  59.3  - 

-     -     -     -  56.2  - 

-     -     -     -  45.8 

70  -     - 

-     -  69.4  - 

-     -     -     -  67.8  - 

....  62 

80  - 

-  80 

-  80 

.  fin 

The  singular  discrepancy  in  the  case  of  water  is  owing  to 
the  peculiarity  described  in  former  pages,  of  its  contracting  by 
cold  only  down  to  40°  of  Fahrenheit,  and  then  dilating  again 
until  it  freezes. 

Laborious  investigations  have  been  made  by  the  French  che- 
mists to  discover  a  comprehensive  law  determining  the  rate  of 
expansion  in  all  bodies;  but  the  object  is  not  yet  satisfactorily 
accomplished. 

e (  To  melt  a  solid  body,  or  to  vaporize  a  liquid,  a  large  quan- 
tity of  heat  enters  it;  but  in  the  new  arrangement  of  the 
particles  and  generally  increased  volume  of  the  mass,  the 
heat  becomes  hidden  from  the  thermometer,  and  is  called 
LATENT  HEAT.  It  reappears  during  the  contrary  changes, 
of ter  whatever  interval."  (See  the  Analysis,  page  13.) 

THE  expansion  of  bodies  by  heat,  instead  of  proceeding 
throughout  in  some  nearly  uniform  or  gradual  manner,  exhibits 
in  its  course  two  singular  transformations  of  the  body,  viz.  when 
the  solid  breaks  down  into  a  liquid,  and  when  the  liquid  swells 
out  into  an  air  or  gas.  The  substance  of  water,  for  instance, 
when  at  a  low  temperature,  exists  in  the  solid  form  called  ice; 
but  at  32°  of  Fahrenheit  it  becomes  liquid  or  water,  and  at  212°, 
even  under  the  resisting  pressure  of  the  atmosphere,  it  sudden- 
ly acquires  a  bulk  nearly  2,000  times  greater  than  it  had  as  a  li- 
quid, being  then  called  steam  or  aeriform  water.  And  other 
bodies  under  analogous  circumstances  undergo  similar  changes. 
It  is  farther  remarkable,  that  although  during  the  changes  a  large 
quantity  of  heat  enters  the  mass,  producing  in  the  one  case  li- 
quidity, in  the  other  the  form  of  air,  the  temperature  is  the  very 
same  immediately  after  as  it  was  before.  Water  running  from 


ti8  HEAT. 

inciting  ice  affects  the  thermometer  but  as  the  ice  does,  and 
steam  over  boiling  water  appears  no  hotter  than  the  water. 
The  glory  of  originally  making  the  discovery  of  the  facts  now 
referred  to  by  the  terms  latent  heat,  or  caloric  of  fluidity, 
belongs  to  the  illustrious  Dr.  Black.  The  construction  of  the 
modern  steam  engine  was  an  early  result  of  kindred  investiga- 
tions, made  by  Dr.  Black's  friend,  James  Watt. 

We  select  the  following  instances  as  serving  to  display  the  sub- 
ject of  latent  heat  in  its  various  bearings. 

A  mass  of  ice  brought  into  a  warm  room,  and  therefore  re- 
ceiving heat  from  every  object  around  it,  will  soon  reach  the 
temperature  of  melting,  or  32°,  but  afterwards  both  the  ice  and 
the  water  formed  from  it  will  continue  at  that  temperature  un- 
til all  be  melted: — the  heat  which  still  continues  to  enter,  ef- 
fecting a  change  only  in  the  form  of  the  mass.  And  in  the  case 
supposed,  whatever  time  was  required  for  heating  the  mass  of 
ice  one  degree,  just  one  hundred  and  forty  times  as  much  will 
be  required  for  inciting  it;  proving  that  140°  is  the  latent  heat 
pf  water. 

If  two  similar  flasks,  one  filled  with  ice  at  32°,  and  the  other 
with  water  at  32°,  be  placed  in  the  same  oven,  or  over  the  same 
flame,  the  water  will  gain  140  degrees  of  heat,  while  the  ice  is 
merely  dissolving  into  water  at  32°:  and  in  the  course  of  the  ex- 
periment, a  correspondence  will  always  exist  between  the  phe^ 
nomena;  for  instance,  when  the  water  has  gained  14°  of  heat,  it 
will  be  found  that  just  a  tenth  part  of  the  ice  is  melted. 

If  equal  quantities  of  hot  and  cold  water  be  mixed  together, 
the  whole  acquires  a  middle  temperature,  each  degree  lost  by  the 
hot  water  becoming  a  degree  gained  by  the  cold:  but  if  a  pound 
of  ice  at  32°,  and  a  pound  of  water  140°  hotter  be  mixed  toge- 
ther, the  140°  of  heat  will  go  merely  to  melt  the  ice;  for  there 
will  result  two  pounds  of  water  at  32°. 

If  a  flask  of  water  at  32°,  or  its  freezing  point,  and  a  similar 
flask  of  strong  brine  at  32°,  but  which  does  not  freeze  until  cooled 
to  near  zero,  be  exposed  together  in  the  same  cold  place,  it  will 
be  found  that  when  the  brine  has  lost  10°  of  its  heat,  the  water 
flask  will  still  exhibit  an  undiminished  temperature,  but  a  four* 


LATENT  HEAT.  69 

teenth  part  of  its  contents  will  be  converted  into  ice.  Now,  as 
in  such  a  case  the  water  flask  must  continue  to  radiate  away  heat 
just  as  much  as  the  other,  it  can  maintain  its  temperature  only 
by  absorbing  into  its  general  mass  the  heat  which  was  latent  in 
the  portion  of  water  frozen. 

It  is  possible,  by  cooling  water  slowly  and  in  perfect  repose, 
to  lower  its  temperature,  while  yet  liquid,  ten  degrees  below  its 
ordinary  freezing  point;  but  then,  on  the  slightest  agitation,  ice 
will  be  formed.  It  might  be  expected  in  such  a  case,  that  the 
whole  water  would  instantly  freeze,  because  all  colder  than  com- 
mon ice;  but  no,  only  a  fourteenth  part  does  so,  and  singularly, 
both  that  fourteenth  and  the  remaining  liquid  are  rendered  in 
the  moment  ten  degrees  warmer.  Here  the  140°  of  latent  heat 
escaping  from  the  fourteenth  part  which  freezes,  become  10°  of 
sensible  heat  for  the  whole  mass,  so  that  the  remaining  water 
has  the  temperature  at  which  it  only  begins  to  freeze. 

Strong  solutions-  in  hot  water  of  various  neutral  salts,  if  al- 
lowed to  cool  while  exposed  to  atmospheric  pressure,  soon  de- 
posit crystals  of  the  salts;  but  in  a  close  vessel  protecting  them 
from  such  pressure,  they  will  remain  liquid  even  when  cold. 
Now,  at  the  moment  of  opening  such  a  bottle,  the  salt  immedi- 
ately crystallizes,  and  the  latent  heat  given  out  by  the  solidify- 
ing particles  warms  very  sensibly  the  remaining  liquid  and  the 
bottle. 

From  the  preceding  facts  it  maybe  perceived,  that  the  quan- 
tity of  ice  formed  or  melted  in  any  case,  becomes  a  correct  mea- 
sure of  the  quantity  of  heat  transferred.  From  this  considera- 
tion, the  illustrious  Lavoisier  constructed  his  calorimeter,  or 
heat  measure.  It  is  a  case  or  vessel  lined  with  ice,  and  the 
quantity  of  heat  given  out  by  any  body  placed  in  it  is  indicated 
by  the  quantity  of  water  collected  from  the  melted  ice. 

Had  the  latent  heat  of  water  been  only  1°  or  2°  instead  of 
140°,  *the  earth,  except  in  its  tropical  regions,  would  have  been 
scarcely  habitable.  The  cold  of  a  single  night  might  have  frozen 
an  ocean,  and  the  heat  of  a  single  day  might  have  converted  the 
accumulated  snows  of  a  winter  into  one  sudden  and  frightful  in- 
undation. As  the  fact  is,  however,  both  changes  are  beautifully 
gradual,  and  easily  controlled  or  prepared  for. 


70  HEAT. 

The  fact  of  latent  heat  in  other  liquids  than  water  is  familiar- 
ly exhibited  in  the  slow  melting — of  the  metals;  lead  or  pig-iron 
for  instance — of  butter  or  oils — of  glass,  &c. ;  and,  on  the  other 
hand,  in  the  slow  solidification  of  the  melted  masses  when  heat 
is  again  abstracted. 

The  substances  below  enumerated,  while  passing  from  the  so- 
lid to  the  liquid  state,  absorb  and  render  latent  the  quantities  of 
heat  here  noted: 

Ice      -  -     140° 

Mercury     -  -142 

Bees'- wax  -  -     170 

Tin     -  442 

Zinc   -  -     492 

If  a  pioce  of  frozen  mercury  (the  temperature  of  which  is  at 
least  40°  below  zero)  be  thrown  into  a  little  water,  the  latent 
heat  of  the  water  immediately  passes  into  the  mercury  and  melts 
it;  but  then  the  water  is  so  cooled  as  to  become  ice. 

"  Latent  heat  of  aeriform  fluids" 

Water  in  a  vessel  placed  over  a  fire  gradually  attains  the  boil- 
ing temperature,  or  212°:  but  its  temperature  then  rises  no  more, 
because  the  farther  addition  of  heat  becomes  latent  in  the  steam 
escaping  during  the  ebullition.  The  quantity  of  heat  which 
becomes  latent  in  steam  is  discovered  by  noting  how  much  more 
time  is  required  for  boiling  a  quantity  of  water  to  dryness,  than 
for  merely  heating  it  a  certain  number  of  degrees.  The  expe- 
riment indicates  about  1,000°;  that  is  to  say,  that  1,000  times 
as  much  heat  is  latent  in  any  quantity  of  water  formed  into 
steam  as  would  raise  the  temperature  of  the  liquid  water  one 
degree.  Watt  had  found  that  water  in  a  vessel  placed  over  a 
lamp  was  about  six  times  as  long  in  being  completely  evapo- 
rated, as  in  being  originally  heated  from  an  ordinary  tempera- 
ture to  that  of  boiling. 

If  we  place  in  the  same  oven  or  over  similar  flames  two  like 
vessels  containing  water,  one  of  which  vessels  is  open  at  top, 
and  the  other  is  strongly  elosed,  the  two  will  gain  heat  equally 
up  to  the  boiling  point,  but  afterwards  the  open  vessel  from 


LATENT  HEAT.  71 

giving  out  steam  will  remain  at  the  same  temperature,  while  the 
other,  by  confining  all  the  heat  that  enters,  will  show  the  tem- 
perature, rising  as  before,  until  the  increasing  tendency,  of  the 
water  to  dilate  forces  the  vessel  open.  Supposing  the  water  in 
the  latter  vessel,  before  vent  is  given,  to  have  become  100°  hot- 
ter than  common  boiling  water,  instead  of  the  whole  being  im- 
mediately converted  into  steam  as  might  be  expected,  only  a 
tenth  part  will  be  so  changed,  (the  same  quantity  as  will  be 
found  to  have  already  escaped  from  the  other  vessel,)  for  the 
tenth  part  requiring  in  the  form  of  steam  1,000°  of  latent  heat, 
will  take  the  excess  of  100°  from  the  other  nine  parts,  and  will 
leave  them  as  common  boiling  water.  If  water,  heated  conside- 
rably beyond  the  boiling  point,  be  allowed  to  expand  very  sud- 
denly, the  whole  is  blown  out  of  the  vessel  as  a  mist,  by  the 
steam  formed  at  the  same  instant  through  every  part  of  the  mass, 
but  the  whole  mass  in  such  a  case  is  no  more  converted  into 
true  steam,  than  the  whole  of  very  brisk  soda  water  is  con- 
verted into  air  when  similarly  thrown  out  by  the  sudden  extri- 
cation of  the  carbonic  acid  gas,  on  uncorking  the  bottle.  Mis- 
conception of  this  matter  has  led  to  most  wasteful  experiments 
on  steam  engines  of  very  high  pressure. 

The  same  indication  of  the  latent  heat  of  steam  is  obtained 
by  the  converse  experiment  of  first  converting  a  quantity  of 
water  into  steam,  and  then  admitting  it  to  cold  water  or  to  ice. 
A  pound  of  steam  will  raise  the  temperature  of  ten  pounds  of 
cold  water  100  degress,  or  will  melt  about  S$  pounds  of  ice. 

In  the  great  quantity  of  heat  which  becomes  latent  in  steam, 
we  perceive  the  reason  why  water  projected  upon  a  raging  fire 
so  powerfully  represses  it: — and  hence,  again,  why  Jlre  and 
water  are  so  often  adduced,  proverbially,  as  furnishing  a  striking 
contrast. 

It  was  when  Watt  had  discovered  how  much  heat  was  lost 
by  losing  steam,  that  he  contrived  the  separate  condenser  for 
his  steam  engine,  by  which  he  saved  three-fourths  of  the  fuel 
formerly  used. 

Substances  differ  among  themselves  in  .regard  to  the  latent 
heat  of  their  vapours  as  much  as  in  their  other  relations  to  heat 
Thus,"  the  latent  heat  of  the  vapour  or  steam  of, 


72  HEAT. 

Water    -  is  -  1,000° 

Vinegar  -         -      900 

Alcohol  -      442 

Ether     -  300 

Oil  of  turpentine  -  177 

From  the  less  latent  heat  in  other  vapours  than  in  that  of  wa- 
ter, we  might  at  first  suppose  that  there  would  be  great  advan- 
tage from  using  them  in  steam  engines.  Accordingly,  numerous 
experiments  have  been  made,  and  patents  secured  under  this 
idea;  but  the  fact  is,  that  in  the  same  proportion  as  the  heat  is 
less,  the  volume  of  the  vapour  is  less,  and  therefore  no  mecha- 
nical advantage  is  obtainable. 

The  influence  of  external  pressure  in  keeping  the  particles  of 
liquids  together  in  opposition  to  the  repulsion  of  heat  seek- 
ing to  render  their  mass  aeriform,  was  considered  in  the  chap- 
ter on  '  Pneumatics?  but  to  make  the  present  section  com- 
plete, the  subject  must  be  here  shortly  resumed. 
Because  any  liquid,  water  for  instance,  while  receiving  heat 
remains  tranquil,  and  apparently  unchanged,  until  it  reaches  the 
boiling  point,  at  which  bubbling  or  conversion  into  vapour  takes 
place,  we  might  suppose  its  ordinary  boiling  temperature  neces- 
sary to  enable  it,  under  any  circumstances,  to  assume  or  to  main- 
tain the  form  of  air.  But  this  is  no  more  true  than  that  a  com- 
mon spring  compressed  against  an  obstacle  has  no  tendency  to 
expand  or  recover  itself  until  the  obstacle  happen  to  give  way. 
Liquid  water,  with  its  heat,  is  really  a  spring  much  compressed 
by  the  weight  of  the  atmosphere,  and  seeking  to  expand  itself 
into  steam  with  force  proportioned  to  its  temperature.  Even 
at  32°,  or  its  freezing  point,  if  placed  in  a  vacuum,  it  assumes 
the  form  of  air,  unless  restrained  by  a  pressure  of  1£  ounce  on 
each  square  inch  of  its  surface:  and  at  any  higher  temperature 
the  restraining  force  must  be  greater:  at  100°,  for  instance,  it 
must  be  13  ounces;  at  150°,  4lbs.;  at  212°,  15lbs.;  at  250°, 
30lbs.,  and  so  on: — and  whenever  the  restraining  force  is  much 
weaker  than  the  expansive  tendency,  the  formation  of  steam 
will  take  place  so  rapidly  as  to  produce  the  bubbling  and  agita- 
tion called  boiling.  Now,  it  is  because  the  atmosphere  or  ocean 


LATENT  HEAT. 


73 


of  air  which  surrounds  the  earth  happens  to  have  in  it  15  Ibs. 
weight  of  air  over  every  square  inch  of  the  earth's  surface,  and 
presses  on  all  things  there  accordingly,  that  212°  is  called  the 
hoiling  point  of  water.  An  atmosphere  less  heavy  would  have 
allowed  liquids  to  burst  into  vapour  at 
lower  temperatures,  and  one  more  hea- 
vy would  have  had  a  contrary  effect. 
The  exact  degree  of  expansive  force  for 
every  degree  of  temperature  in  water 
and  other  liquids,  has  been  ascertained 
by  heating  them  in  vessels  furnished 
either  with  properly  loaded  valves,  as 
at  f  in  this  figure,  or  with  a  tall  up- 
right tube,  as  d  b,  into  which  the  liquid, 
c9  may  force  a  column  of  mercury  to 
an  elevation  marking  the  expansive  ten- 
dency; the  valve  and  mercury  being  of 
course  protected  from  the  external  at- 
mospheric pressure,  or  the  necessary  allowance  being  made  for 
that  pressure.  Boiling  at  the  bottom  of  a  deep  vessel  is  re- 
sisted by  the  weight  of  the  liquid  in  addition  to  that  of  the  at- 
mosphere, as  already  explained,  and  consequently  the  tempe- 
rature at  which  it  occurs  is  there  higher  than  near  the  surface 
of  the  vessel.  Boiling  heat  is  greater  also  in  other  cases,  as — 
in  a  deep  mine,  where  of  course  there  is  additional  depth  and 
weight  of  atmosphere  over  any  exposed  liquid, — at  times  when 
the  'barometer  is  unusually  high,  that  is  to  say,  when  the  at- 
mosphere is  unusually  heavy — in  cases  where  air  or  stearn  is 
confined  over  the  boiling  surface  so  as  to. press  more  upon  it, 
as  when  brewers  for  a  time  shut  the  lid  or  valve  of  their  great 
boilers,  &c.  Water  placed  on  the  fire  in  a  strong  vessel,  from, 
which  steam  cannot  at  all  escape,  may  be  rendered  even  red 
hot,  without  a  bubble  forming  or  one  particle  being  dissipated; 
but  the  tendency  to  expand  into  steam  is  then  great  enough  to 
burst  any  known  material  of  moderate  thickness.  The  Mar- 
quis of  Worcester  exploded  a  cannon  by  shutting  up  water  in 
it,  and  then  surrounding  it  with  fire.  Boiling  temperature  is 
lower  again  when  the  experiment  is  made  on  mountains  or  in 

10 


74  HEAT. 

other  situations  above  the  level  of  the  sea,  where  there  is  less 
height  of  air  resting  over  the  boiler.  In  the  city  of  Mexico, 
which  is  7,000  feet  above  the  sea,  water  boils  before  it  reaches 
the  heat  of  200°,  instead  of,  as  in  places  near  the  sea-level,  at 
212°.  Wollaston's  thermometer,  beautifully  adapted  for  de- 
termining the.  height  of  mountains,  balloon  ascents,  &c.,  by 
merely  indicating  the  heat  of  boiling  water  in  any  situation,  is 
a  fine  illustration  of  this  truth.  If,  in  any  place,  we  take  off 
the  atmospheric  pressure  from  a  liquid,  as  by  placing  it  in  the 
receiver  of  an  air  pump,  it  will  boil  at  very  low  temperatures 
indeed.  Water  thus  treated  boils  at  70°,  which  is  20°  below 
the  heat  of  some  English  summer  days,  and  ether  boils  when 
colder  than  common  ice.  Generally,  in  a  vacuum,  substances 
boil  at  a  temperature  124°  lower  than  while  restrained  by  the 
atmospheric  pressure. 

Consequences  of  these  truths,  respecting  the  boiling  tempera- 
ture, are  the  following: — 

As  water  when  heated  dilates  itself  in  the  form  of  steam, 
the  steam  presses  on  a  given  extent  of  surface  with  the  same 
force  as  the  water  itself  would  do;  and  in  a  steam  engine,  the 
temperature  of  the  water  tells  the  degree  of  force  which  the 
steam  is  exerting  on  the  piston. 

Because  in  the  case  of  steam,  the  same  law  holds  as  for  aeri- 
form fluids  generally,  viz.  that  the  outward  elasticity  or  spring 
increases  in  proportion  as  the  fluid  is  more  condensed — high- 
pressure  steam  is  merely  condensed  steam,  just  as  high-pres- 
sure air  is  condensed  air;  and  to  obtain  a  double  or  triple  pres- 
sure, we  must  have  twice  or  thrice  the  quantity  of  steam  un- 
der the  same  volume. 

The  reason  that  high-pressure  steam,  issuing  from  a  boiler 
heated  perhaps  to  300°,  is  not  hotter  than  low-pressure  steam 
from  a  boiler  at  212°,  is,  that  in  the  instant  when  the  high- 
pressure  or  condensed  steam  escapes  into  the  air,  it  expands 
until  balanced  by  the  pressure  of  the  atmosphere,  that  is,  un- 
til it  become  low-pressure  steam,  and  it  is  cooled  by  the  ex- 
pansion, as  air  is  cooled  on  escaping  from  any  condensation. 

The  vessel  called  Papin's  Digester,  is  merely  a  pot,  which 


BOILING.  75 

can  be  kept  closed  in  spite  of  the  force  of  the  steam  formed 
within  it;  and  in  such  a  vessel  water  can  be  heated  far  beyond 
the  ordinary  boiling  point, — enough,  for  instance,  to  dissolve 
and  extract  all  the  gluten  or  jelly  of  bones,  and  to  form  from 
them  a  rich  soup  where  common  boiling  would  procure  no- 
thing:— or  even  enough  to  melt  lead. 

The  person  who  urges  a  fire  under  a  boiling  pot  with  the 
hope  of  making  the  water  hotter,  is  foolishly  wasting  the  fuel; 
for  the  water  can  only  boil,  and  it  does  so  at  212°  of  the  ther- 
mometer. 

As  different  substances  under  a  given  pressure,  become  aeri- 
form at  different  temperatures,  mixtures  of  such  may  be  de- 
composed by  heat.  If  a  mixture  of  spirit  and  water,  for  in- 
stance, be  placed  over  a  fire,  the  spirit  will  boil  off  long  be- 
fore the  water.  If  the  spirituous  vapour  be  caught  apart  and 
condensed,  it  is  said  to  have  been  distilled.  Other  distillations 
are  of  the  same  nature. 

The  instrument  here  represented 
consists  of  a  glass  tube  blown  into 
kulDS  at  tne  two  ends  a  and  b,  and 
hermetically  sealed,  after  receiving 
into  it  some  water,  but  no  air.  If  one  of  the  bulbs  be  heated 
more  than  the  other,  the  steam  or  vapour  in  that  one  will,  for 
the  reasons  stated  above,  be  denser  and  stronger  than  in  the 
other,  and  will,  therefore,  force  its  way  into  the  other;  but, 
owing  to  the  lower  temperature  there,  a  part  of  it  will  be  con- 
densed, making  room  for  more.  Hence,  if  the  difference  of 
temperature  between  the  bulbs  be  long  maintained,  the  whole 
water  will  by  a  sort  of  distillation  gradually  pass  into  the  colder 
bulb.  If  the  difference  of  temperature  become  at  any  time  con- 
siderable, the  liquid  will  boil  in  the  warmer  bulb,  even  although 
the  source  of  heat  be  only  the  living  hand  grasping  it. 

To  the  author  it  appears  that  by  a  larger  apparatus  made  on 
this  principle,  fresh  water  might  be  conveniently  obtained  from 
salt  water  on  board  ship,  or  on  an  island  having  no  fresh  springs. 
Suppose  any  two  air-tight  vessels  like  a  and  b,  communicating 
by  a  tube  furnished  with  a  stop-cock  near  b,  then  if  the  vessel, 
«,  were  filled  with  salt  water,  and  were  heated  by  being  ex- 


76  HEAT. 

posed  to  the  sun,  (its  surface  being  blackened  and  protected  by 
glass  from  the  cooling  effect  of  the  air,)  and  if  the  other  ves- 
sel, by  after  being  also  filled  with  water,  were  made  a  vacuum 
by  pumping  the  water  out  from  the  bottom,  and  were  then 
kept  as  cold  as  possible  by  wetted  coverings  and  a  current  of 
air,—r-on  opening  the  cock  at  b,  vapour  would  pass  over  from 
the  heated  vessel  to  be  constantly  condensed  in  the  colder,  and 
there  would  be  a  distillation  from  the  sea  water  of  perfectly 
sweet  water  by  the  natural  action  of  the  sun  alone.  Cases 
have  occurred  where  a  knowledge  of  this  fact  would  have  saved 
ship-wrecked  crews  from  perishing  by  thirst;  and  there  are 
rocky  islands  in  the  ocean  which  would  become  pleasantly  ha- 
bitable by  the  adoption  of  such  a  means,  where  now  there  is  no 
supply  of  fresh  water,  but  from  precarious  rain  or  importation 
from  abroad. 

When  a  substance  has  reached  the  temperature  at  which  it 
boils,  that  is  to  say,  at  which  its  vapour  becomes  a  balance  to 
the  atmospheric  pressure,  its  dilating  force  is  strong  indeed. — 
Persons  may  not  reflect  that  15  Ibs.  on  a  square  inch  is  about  a 
ton  on  a  square  foot, — and  such  is  the  power  with  which  the 
vapour  of  all  boiling  substances  rises  from  them — sufficient  in 
a  single  Cornish  steam  engine  to  urge  the  piston  with  the  power 
of  600  horses!  But  the  tendency  to  expand  at  temperatures 
much  below  boiling,  is  still,  as  already  stated,  very  great,  and 
although  not  attracting  common  attention,  is  silently  working 
many  beautiful  and  important  ends  in  the  economy  of  nature. 
As  into  a  perfect  vacuum,  freezing  water  gives  out  a  steam  or 
vapour  that  would  lift  with  force  of  li  ounce  per  inch,  or  16 
Ibs.  on  a  square  foot,  and  even  solid  ice  gives  out  its  vapour  of 
nearly  equal  strength;  so  also  do  other  liquids  and  solids.— 
There  is  an  aeriform  mercury,  dense  in  proportion  to  the  tem- 
perature, in  the  apparently  empty  space  called  the  Torricellian 
vacuum,  over  the  mercury  in  a  barometer  tube;  and  around 
camphor,  all  the  essential  or  volatile  oils,  &c.,  there  is  similar- 
ly an  atmosphere  of  the  substance  in  the  form  of  air. 

It  had  for  a  considerable  time  been  known  that  into  a  per- 
fect vacuum  bodies  emitted,  almost  instantly,  in  the  form  of 
air,  a  quantity  of  their  substance  proportioned  to  their  tempe- 


EVAPORATION.  77 

rature;  but  it  was  reserved  for  Mr.  Dalton  to  make  the  ad- 
mirable discovery,  that  even  into  any  space  filled  with  air 
these  vapours  arise  in  quantity  and  density  the  same  as  if  air 
were  not  present — the  two  fluids  seeming  to  be  independent  of 
each  other,  with  the  exception  that  in  a  vacuum  the  equal  dif- 
fusion of  a  vapour  takes  place  at  once,  while  in  a  situation  al- 
ready occupied  by  air,  it  proceeds  only  as  the  vapour  can  force 
its  way  through  the  particles  of  the  air,  and  in  general  takes 
place  by  a  tranquil  evaporation  from  the  surface  instead  of  the 
agitation  of  ebullition.  In  an  apartment  with  an  open  vessel 
of  water  in  it,  there  is  soon,  although  invisible,  a  steam  of  wa- 
tery vapour  mingled  with  the  air,  as  dense  as  if  the  room  were 
a  vacuum  at  the  same  temperature. 

Consequences  of  this  important  truth  are  the  following: — 

That  it  is  only  an  atmosphere  of  the  substance  of  each  body, 
which  by  pressing  on  it  can  prevent  its  farther  dissipation  by 
heat.  Thus  we  can  only  save  camphor,  musk,  smelling  oils, 
spirits,  water,  &c.,  by  placing  them  in  closed  bottles  or  ves- 
sels, in  which,  additionally  to  the  air  present,  an  atmosphere 
of  their  ewn  substance  is  soon  formed,  involving  the  remain- 
ing masses  with  pressure  proportioned  to  their  temperature  and 
its  density. 

The  important  process  of  drying  things  is  merely  the  placing 
them  under  an  elevated  temperature  if  attainable,  and  in  an  at- 
mosphere not  containing  so  much  of  the  liquid  as  to  be  satu- 
rated at  the  temperature.  The  effect  of  wind  or  motion  of  the 
air  in  quickening  evaporation,  is  owing  to  its  removing  air  sa- 
turated with  the  moisture,  and  substituting  air  which  is  not — 
thus  producing  nearly  the  case  of  the  substance  placed  in  a  va- 
cuum. 

If  air,  at  a  certain  temperature,  contain  mixed  with  it  as 
much  water  as  can  be  sustained  in  the  form  of  invisible  vapour 
at  that  temperature,  and  if  by  any  cause,  as  by  rising  in  the  at- 
mosphere, the  air  be  then  cooled,  it  will  abstract  heat  from  the 
vapour,  and  cause  a  portion  of  this  to 'be  precipitated  or  visi- 
bly condensed  into  a  fog  or  rain.  Water  rising  as  invisible 
vapour  from  the  surface  of  a  lake  or  river,  when  it  has  readied 


78  HEAT. 

a  certain  height,  is  condensed  into  the  stratum  of  clouds,  which 
at  times  usefully  protect  the  fields  from  the  intense  meridian 
sun,  and  may  fall  again  as  refreshing  showers  over  the  country. 

It  is  the  tranquil  and  invisible  evaporation  of  which  we  are 
now  speaking,  which  lifts  from  the  surface  of  the  wide  ocean 
all  the  water  which,  after  condensation,  is  again  returning  to 
it  in  the  myriads  of  river  streams  which  give  life  and  beauty 
to  the  face  of  nature. 

In  warm  climates  there  are  inlets  of  the  sea,  shut  off  occa- 
sionally from  the  parent  ocean,  and  where,  after  the  sun's  rays 
have  drank  up  all  the  water,  the  deposited  salt  remains  to  be 
carried  away  in  loads  for  the  uses  of  man,  as  sand  is  carried 
from  any  ordinary  shore.  There  are  in  the  bowels  of  the 
earth  prodigious  accumulations  of  salt,  formed,  doubtless,  in 
the  same  way,  during  the  revolutions  of  the  antediluvian  world, 
and  now  explored  as  salt  mines.  When  the  Nile  overflows  its 
banks  with  waters,  dissolving,  although  in  almost  impercepti- 
ble proportion,  mineral  substances  brought  from  central  Africa, 
and  fills  reservoirs  afterwards  dried  up  by  the  sun's  he.at,  it 
leaves  in  these  a  rich  store  of  crystallized  natron  or  soda. 

The  following  are  other  instances  of  vapour,  invisible  while 
at  a  higher  temperature,  being  thickly  precipitated  when  air, 
with  which  it  is  mixed,  is  cooled,  or  when  it  touches  a  colder 
solid  body: — the  steam  observed  at  night  and  morning  hover- 
ing over  brooks  and  marshes  heated  by  the  sun  during  the 
day: — the  frost-smoke,  as  it  is  called,  which  lies  on  the  whole 
face  of  the  Greenland  seas  in  the  beginning  of  winter,  where 
the  water  warmed  by  the  long  day  of  the  polar  summer,  con- 
tinues to  emit  its  vapour  for  a  considerable  time  after  summer 
is  past,  into  an  atmosphere  become  too  cold  to  preserve  it  in- 
visible:— the  breath  or  perspiration  of  animals,  of  horses  in  par- 
ticular after  strong  exertion,  becoming  so  strikingly  visible  in 
cold  and  damp  weather,  or  even  in  warm  weather,  when  the 
air  is  already  charged  with  moisture: — in  cities  where  there 
are  deep  drains  communicating  with  kitchens,  manufactories, 
&c.,  and  constantly  filled  with  moist  and  warm  air;  the  vapour- 
loaded  air,  although  clear  or  transparent  below,  immediately 
on  escaping  into  a  frosty  atmosphere,  lets  go  its  moisture,  with 


EVAPORATION.  79 

the  appearance  of  steam  issuing  from  a  great  subterranean  caul- 
dron. Steam  over  water  in  any  boiler  is  transparent  or  per- 
fectly aeriform — as  may  be  seen  when  water  is  made  to  boil  in 
a  vessel  of  glass,  but  as  soon  as  it  is  cooled  by  contact  or  ad- 
mixture of  colder  air  it  ceases  to  be  true  steam,  and  is  con- 
densed into  small  particles  of  water  suspended  in  the  air. 
Many  persons,  while  thinking  of  steam,  figure  it  only  in  this 
latter  state,  as  particles  of  water  mixed  with  air  nearly  as  a  sub- 
tle powder  might  be  mixed,  and  its  substance  occupying  really 
no  more  space  than  the  original  water  did.  Now  until  steam 
is  cooled  and  condensed,  it  is  of  a  nature  to  fill  alone  any  ap- 
propriate vessel  and  powerfully  distend  it,  just  as  air  fills  and 
distends  a  bladder.  Steam  issuing  from  the  spout  of  a  kettle 
is  hardly  seen  near  the  mouth,  but  as  its  distance  from  the 
spout  increases,  it  is  cooled  into  a  thick  cloud  or  vapour. 

In  a  vessel  from  which  air  and  atmospheric  pressure  are  ex- 
cluded, even  the  temperature  of  freezing  water  being  sufficient  to 
maintain  permanently  in  the  state  of  gas  or  air,  many  substances 
which  exist  as  liquids  under  the  atmospheric  pressure, — and 
the  whole  mass  of  such  a  substance  when  placed  in  a  vacuum, 
not  being  instantly  converted  into  gas,  because  the  portion  which 
first  rises  becomes  an  atmosphere  weighing  upon  the  remaining 
mass,  and  because,  moreover,  that  portion,  by  absorbing  from 
the  mass  much  heat  into  the  latent  state,  cools  the  mass  much  be- 
low the  freezing  point; — we  see  why  the  liquids  now  spoken 
of  are  so  rapidly  cooled  to  at  least  the  freezing  point  if  placed 
where  a  vacuum  can  be  maintained,  that  is.  to  say,  where,  after 
common  air  has  been  removed,  the  aeriform  matter  rising  from 
them,  and  absorbing  their  heat,  is  promptly  and  in  a  continued 
manner  abstracted.  It  is  thus  that  water  placed  in  the  exhausted 
receiver  of  an  air-pump  is  so  rapidly  cooled,  and  that  when 
there  is  beside  it  a  vessel  of  concentrated  sulphuric  acid,  or 
other  substance  capable  of  absorbing  the  watery  vapour  as 
formed,  it  is  soon  reduced  to  the  state  of  ice;  or  again,  that 
water,  or  even  mercury,  surrounded  by  ether  evaporating  in  a 
vacuum,  is  so  quickly  frozen.  It  is  thus  also  that  if  one  bulb 
of  the  instrument  described  at  page  75  be  immersed  in  a 
freezing  mixture,  the  water  in  the  other  and  distant  bulb  will 


SO  HEAT. 

soon  become  ice;  for  the  vapour  rising  from  that  water  into  the 
vacuum  maintained  throughout  the  apparatus  by  the  freezing 
mixture,  is  immediately  condensed  again  in  the  immersed  bulb, 
and  leaves  the  vacuum  still  free  for  the  ascent  of  more  vapour, 
to  carry  away  more  heat  from  the  water  as  latent  heat,  and  to 
make  it  freeze. 

As  we  have  explained  also,  that  in  a  liquid  there  is  the  same 
tendency  to  evaporate  whether  it  be  or  be  not  exposed  to  the 
air,  we  see  the  reason  why  all  evaporation  is  a  very  cooling 
process.  The  effect  however  in  air,  is  neither  so  rapid  nor  so 
great  as  in  a  vacuum;  first,  because  the  presence  of  the  air  im- 
pedes the  spreading  from  the  liquid  surface  of  the  newly  formed 
vapour,  and  keeps  it  where  its  pressure  resists  the  formation  of 
more  vapour;  and,  secondly,  because  the  air  in  contact  with 
the  liquid,  shares  its  higher  temperature  with  the  liquid.  Still 
in  India  flat  dishes  of  water,  placed  through  the  night  on  beds 
of  twigs  and  straw  kept  wet  and  in  a  current  of  air,  soon  ex- 
hibit thin  cakes  of  ice — and  thus  ice  is  procured  in  India  for 
purposes  of  luxury. 

The  absorption  of  latent  heat  in  the  evaporation  which  goes 
on  from  the  sea  and  earth  in  all  warm  climates,  greatly  tempers 
the  heat  of  these  climates,  and  the  vapour  afterwards  spreading 
to  the  poles,  as  explained  in  'Pneumatics?  under  the  head  of 
winds,  carries  warmth  thither  to  be  given  out  when  it  is  re- 
condensed  into  the  form  of  rain,  or  is  solidified  as  snow.  The 
formation  any  where  of  mist  or  rain  warms  the  air  most  sensi- 
bly, by  the  liberation  of  the  latent  heat  from  the  precipitated 
vapour.  Again,  the  liquid  water  which  during  winter  is  con- 
verted into  snow  or  ice  had  been  a  reservoir  of  latent  heat 
stored  to  temper  the  frosty  air  of  the  commencing  cold  season; 
and  in  the  following  spring,  such  ice  and  snow  serves  as  empty 
receptacles  in  which  the  first  violence  of  the  returning  sun  hides 
or  expends  itself;  allowing  the  temperature  to  change  more  gra- 
dually, and  for  many  living  beings  therefore  more  safely.  The 
vast  stores  of  ice  and  snow  among  high  mountains,  as  among 
the  Alps  and  Pyrenees,  are  often  stores  of  mild  temperature  to 
regions  around;  for  besides  cooling  the  air  near  them,  they  are 
the  never-failing  sources  of  the  rivers  which  run  from  them 


EVAPORATION.  81 

during  the  whole  of  summer,  carrying  freshness  through  the 
lands; — from  the  Alps,  for  instance,  proceed  the  Rhine  and 
Rhone — most  romantic  and  beautiful  of  European  streams;  and 
from  the  Pyrenees,  the  little  Gave,  &c.,  which  while  channels 
around  from  lower  regions  are  almost  dried  up  by  the  summer 
heat,  flows  only  the  more  freshly  as  the  heat  is  greater^  and  the 
feeding  snows  are  more  abundantly  dissolved. 

Men  in  artificially  raising  temperatures  are  generally  causing 
the  liberation  of  heat  which  had  been  previously  latent,  and 
in  lowering  temperature  or  producing  cold,  they  almost 
solely  effect  their  purpose  by  rendering  a  quantity  of  heat 
latent. 

Lavoisier  thought  that  the  heat  of  all  combustion  was  merely 
the  latent  heat  of  the  oxygen  gas  concerned  in  the  combustion, 
given  out  during  its  combination  with  the  burning  body.  It  ia 
so  in  part,  but  we  now  know  that  it  depends  more  on  the  Inteii* 
sity  of  the  chemical  action  between  the"  combining  substances. 
The  water  thrown  upon  quicklime  to  slakd  it,  becomes  solid 
in  combination  with  the  lime,  and  gives  out  its  latent  heat  so 
remarkably  as  often  to  set  fire  to  a  Wooden  Vessel  or  ship  con- 
taining it. 

When  dwelling-houses,  green  houses,  manufactories,  &c.  arc 
warmed,  as  is  now  common,  by  the  admission  of  steam  into 
systems  of  pipes  which  branch  over  them,  the  heat  is  chiefly 
that  lately  latent  in  the  steam^  and  which  spreads  around  as 
soon  as  the  steamy  by  touching  pipes  of  lower  temperature,  is 
condensed  to  a  state  of  Water.  The  modes  of  most  profitably 
effecting  these  purposes  have  to  be  considered  in  a  future 
chapter. 

For  producing  artificial  coldy  our  processes  generally  involve 
the  circumstance  either  of  a  solid  changing  into  a  liquid,  during 
which  it  absorbs^  and  hides  in  its  new  constitution  much  of  the 
heat  previously  sensible  in  it  and  in  the  liquid  dissolving  it,  or 
of  a  liquid  changing  into  vapottr,  during  which  heat  equally  be- 
comes latent.  Thus  by  dissolving  a  salt,  nitre  for  instance,  in 
water,  we  obtain  a  solution  very  cold. 

In  India  the  common  mode  of  cooling  \vine  for  table  is  to 

11 


83  HEAT. 

surround  the  bottles  with  nitre  thus  melting;  and  the  water  of 
the  solution  being  evaporated  again  before  next  day,  the  salt  is 
left  ready  for  use  as  before.  Such  is  the  mutual  attraction  of 
water  and  many  salts,  that  they  readily  combine  with  form  of 
liquid,  even  when  the  water  is  used  in  the  solid  state  of  ice; 
and  as  both  the  water  and  the  salt  then  make  heat  latent,  the 
fall  of  temperature  is  very  great.  Thus  common  salt  and  snow 
mixed,  dissolve  into  liquid  brine  37°  colder  than  freezing 
water,  or  5°  below  the  zero  of  Fahrenheit. 

The  following  is  a  short  table  of  easily  procured  freezing 
mixtures: 

Frigorific  Mixtures. 

Substances  mixed.  Thermometer  sinks. 

Common  salt Ipart  7  From  any  temperature  to  5°  below 

Snow,  or  pounded  ice 2  —  5     zero- 
Common  salt ,o~~?  From  any  temperature  to  25°  below 

Snowonce 12  —  J>  J 

__. .            /»               •  f  zero* 

Nitrate  of  ammonia 5  —  j 

Snow. . 3  —  7  F        320    b         t    23o  bdow  zero< 

Diluted  sulphuric  acid 2  —  5 

Fusedpotass 4—7  p        32o  ab         to  51o  below  zero> 

Snow 3  —  5 

Mtrate  of  ammoma. .............  1  - 

Sulphate  of  soda 8-7  5QO      Q0 

Muriatic  acid 5  —  5 

We  have  already  described  under  other  heads  the  frigorific 
effect  of  evaporating  in  a  vacuum  or  in  the  air,  and  of  the  ope- 
ration of  condensing  a  gas  to  squeeze  the  heat  out  of  it  before 
letting  it  expand  again  to  a  great  volume. 

For  any  given  substance,  the  changes  of  state  from  solid  to 
liquid,  and  from  liquid  to  air,  happen  under  similar  cir- 
cumstances, so  precisely  at  the  same  temperature,  that 
they  mark  fixed  points  in  a  general  scale  of  temperature, 
and  enable  us  to  regulate  and  compare  our  various  ther- 
mometers. (See  the  Analysis,  p.  13.) 

As  we  can  neither  weigh  heat,  nor  measure  its  bulk,  nor  see 
it,  and  as,  even  if  our  sense  of  touch  were  a  correct  judge  in 
the  matter,  which  it  is  not,  we  dare  not  touch  things  that  are 


EVAPORATION. 


83 


very  hot  or  cold,  some  other  means  was  wanted  for  estimating 
the  presence  in  bodies  of  this  very  subtle  principle: — and  a 
means  has  been  found  in  the  measurement  of  its  most  obvious 
and  constant  effect,  namely,  that  dilatation  or  expansion  of  bo- 
dies which  again  ceases  when  the  heat  is  withdrawn.  Any 
substance  so  circumstanced  as  to  allow  this  expansion  to  be  ac- 
curately measured,  becomes  to  us  a  thermometer  or  measure 
of  heat. 

In  solid  substances,  the  direct  expansion  by  heat  is  so  small 
as  to  be  seen  or  measured  with  difficulty.  In  airs,  again,  the 
expansion  is  very  extensive,  but  there  is  the  objection  that  in 
any  apparatus  yet  contrived,  which  will  allow  their  expansion 
completely  to  appear,  they  cannot  be  protected  from  the  vary- 
ing pressure  of  the  atmosphere— an  influence  which  affects 
their  volume  even  more  than  common  changes  of  temperature. 
But  liquids  are  free  from  both  disadvantages, 
and  when  placed  in  a  glass  bulb,  as  ay  having 
a  long  neck  or  stalk  a  b  proceeding  from  it, 
into  which  the  liquid  may  rise  when  expand- 
ed by  heat,  to  be  measured,  they  form  the 
most  generally  convenient  of  thermometers. 
Then,  among  liquids,  mercury  stands,  on 
several  accounts,  singularly  pre-eminent:  in 
it  the  range  of  temperature  between  freezing 
and  boiling  reaches  a  higher  point  than  in 


B- 


F- 


any  other  liquid,  and  a  lower  than  in  all  others  except  alcohol; 
its  little  capacity  for  heat,  and  ready  conducting  power,  cause 
it  to  be  very  quickly  affected  by  change  of  temperature;  its  ex- 
pansion is  singularly  equable  for  equal  increase  of  heat  through 
the  important  middle  part  of  the  scale,  which  includes  the  com- 
mon temperatures  on  earth,  or  from  the  freezing  to  the  boiling 
heat  of  water;  and  it  is  easy  to  proportion  the  bulb  and  the 
stalk  to  each  other,  so  that  a  small  difference  of  temperature 
shall  cause  the  mercurial  column  in  the  stalk  to  rise  or  fall  very 
conspicuously. 

Now,  when  the  important  fact  was  ascertained  that  ice  melts 
in  every  case  at  precisely  the  same  temperature,  and  that  pure 
water  in  a  metallic  vessel;  and  under  a  given  atmospheric  pres- 


84  HEAT. 

sure,  boils  always  at  the  same  temperature,  it  followed,  that  by 
placing  such  a  thermometer  as  above  described,  first  in  melting 
ice,  and  then  in  boiling  water,  and  marking  upon  the  stalk  the 
two  points  at  which  the  mercury  stood,  viz.  F.  and  B.,  two 
fixed  or  invariable  points  would  be  obtained,  and»the  interval 
between  them  might  be  divided  on  the  glass,  or  on  a  suitable 
scale  to  be  attached  to  the  glass,  into  any  convenient  number  of 
parts  to  be  called  degrees:  it  followed  farther,  that  by  continuing 
the  divisions  to  any  extent  both  above  and  below  the  fixed 
points,  a  general  scale  of  temperature  would  be  obtained,  with 
respect  to  which  all  thermometers  made  on  the  same  principle 
would  perfectly  agree,  although  the  size  of  the  divisions  on  the 
stalks  would  vary  according  to  the  comparative  capacities  of  the 
bulb  and  stalk  in  the  different  instruments.  Our  Newton  had 
the  honour  first  to  propose  the  regulating  points  of  freezing  and 
boiling,  and  they  are  now  universally  adopted;  but  the  inter- 
val between  them  has  been  variously  subdivided; — >that  is  to 
say,  there  has  not  been  agreement  among  philosophers  as  to  what 
should  be  accounted  a  degree  of  heat.  In  the  Centigrade  ther- 
mometer, which  is  the  most  simple,  the  division  is  into  100 
equal  parts;  in  Reaumur's,  which  is  commonly  used  in  France, 
it  is  into  80  parts;  and  in  Fahrenheit's,  which  is  used  in  England, 
it  is  into  180°.  In  Fahrenheit's,  moreover,  the  freezing  point, 
instead  of  being  called  zero,  as  in  the  others,  is  called  32°,  be- 
cause the  maker  chose  to  begin  counting  from  the  lowest  heat 
which  he  met  in  Iceland,  or  33  below  freezing  of  his  scale. — 
To  turn  the  degrees  of  any  one  of  these  thermometers  into  de- 
grees of  any  other,  we  have  only  to  recollect  that  9°  of  Fahren- 
heit are  equal  to  5°  of  the  Centigrade,  and  to  4°  of  Reaumur. 
Therefore,  multiplying  by  9  and  dividing  by  5  or  4,  or  the  re- 
verse, adding  or  subtracting  the  32°  of  Fahrenheit,  gives  as  the 
result  the  degree  desired, 

The  bulb  of  a  mercurial  thermometer  is  formed  by  heating  in 
*  lamp  to  fusion  the  end  of  a  glass  tube,  which  has  a  very  small 
and  equable  bore,  and  then  blowing  into  the  tube  until  the  soft- 
ened end  swells  like  a  soap  bubble  to  the  size  desired.  The 
mercury  is  forced  into  such  a  bulb  through  its  long  stalk  by  the 
pressure  of  the  atmosphere  jit  two  efforts.  First,  a  portion  of 


THERMOMETERS.  85 

the  air  originally  in  the  bulb  being  expelled  by  warming  the 
bulb,  the  open  end  of  the  stalk  is  immersed  in  mercury,  and 
when  the  air  still  remaining  in  the  bulb  cools  and  contracts,  a 
little  mercury  enters.  Secondly,  this  admitted  mercury  having 
been  made  to  boil,  so  as  to  fill  with  its  vapour  instead  of  the  air, 
the  whole  capacity  of  the  bulb  and  tube,  on  the  open  end  being 
again  immersed  in  mercury,  and  the  mercurial  vapour  within 
being  condensed,  the  atmosphere  presses  in  fresh  mercury  to 
fill  the  whole  vacuum.  To  complete  the  making  of  the  ther- 
mometer, the  bulb  is  again  heated  to  expel  so  much  of  the  mer- 
cury as  that  when  cold  the  tube  shall  be  about  one-third  full  of 
it,  and  then  before  the  heated  mercury  begins  to  recede,  the  end 
or  opening  is  permanently  closed  by  directing  upon  it  the  point 
of  a  blow-pipe  flame. 

Although  the  direct  expansion  of  any  solid  body  by  a  mode- 
rate change  of  temperature  is  so  inconsiderable  as  to  be  with  dif- 
ficulty measured,  M.  Breguet,  of  Paris,  lately  with  much  in- 
genuity contrived  a  thermometer  in  which  the  index  is  moved 
by  the  curling  or  bending  of  a  solid  when  heated,  as  when  a 
sheet  of  damp  paper  curls  on  being  held  before  the  fire.  Having 
soldered  side  by  side  two  very  small  flattened  wires  of  silver  and 
platinum,  or  of  any  other  metals  having  different  expansibility 
by  heat,  he  found  that  all  changes  of  temperature  made  such  com- 
pound wires  bend  to  a  great  extent,  the  metal  most  shortened 
or  least  lengthened  acting  like  a  bow-string  to  pull  the  other  into 
the  arched  form:  he  then,  by  giving  to  a  compound  wire  a  spiral 
or  cork-screw  form,  and  fixing  the  upper  end  of  it  to  a  stand, 
found  that  an  index  like  the  hand  of  a  watch  connected  with  the 
lower  end  was  turned  completely  round  by  a  certain  change  of 
temperature,  and  that  when  a  circle  of  degrees  were  marked  on 
a  plate  like  a  watch  face  placed  below  the  index,  the  indications 
of  the  instrument  perfectly  agreed  with  those  of  good  mercurial 
thermometers.  Other  modifications  of  the  same  principle  have 
since  been  successfully  tried,  so  simplified  and  reduced  in  bulk  as 
to  be  introduced  into  the  structure  of  a  pocket  watch. 

Air  is  a  substance  on  several  accounts  admirably  adapted  to 
the  formation  of  a  thermometer;  for  it  has  great  extent  of  dila- 
tation from  small  increase  of  heat;  it  quickly  receives  impression, 


86 


HEAT. 


and  its  dilatation  is  equal  for  equal  increments  of  heat  at  all  tem- 
peratures:— but,  as  already  stated,  there  is  the  strong  objection 
that  the  pressure  of  the  atmosphere  cannot  be  excluded,  without 
at  the  same  time  confining  the  air,  and  affecting  its  expansion. 
Mr.  Leslie,  however,  has  used  for  particular  purposes  an  air 
thermometer,  which  he  calls  the  differential  thermometer.  It 
consists  of  two  bulbs,  a  and  b,  filled  with 
air,  and  connected  by  a  be*nt  tube,  d  c,  con- 
taining liquid,  the  bulbs  being  hermetically 
sealed,  so  that  the  atmosphere  cannot  affect 
the  air  within.  The  difference  of  heat  in  one 
and  in  the  other  is  marked  by  the  descending 
of  the  liquid  in  one  of  the  tubes,  d,  which 
has  a  scale  attached  to  it.  We  may  observe, 
that  equal  divisions  or  degrees  marked  on  the 
scale  of  this  thermometer  cannot  mark  equal 
changes  of  temperature,  as  the  increasing  con- 
densation and  resistance  of  the  air  in  one 
bulb  requires  the  force  overcoming  it  progressively  to  increase. 
If  the  resistance,  on  the  contrary,  were  unvarying,  as  in  an  air 
thermometer,  open  to  a  steady  atmosphere,  equal  extent  of  mo- 
tion in  the  fluid  would  mark  equal  increments  of  heat.  An  air 
thermometer  made  of  a  simple  bulb  and  long  stalk  of  semi-trans- 
parent porcelain,  and  containing  in  its  neck  melted  lead  or  other 
fusible  metal  instead  of  mercury,  and  with  the  mouth  down- 
wards, is  well  adapted  for  measuring  very  high  temperatures. 

Temperatures  below  that  of  freezing  mercury  are  usually 
measured  by  alcohol,  as  being  a  substance  which  has  not  yet 
been  frozen:  and  temperatures  higher  than  that  of  boiling  mer- 
cury are  measured  by  the  expansion  of  air  or  of  metals,  as 
above  described,  or  by  the  contraction  of  pieces  of  baked  clay, 
which  when  highly  heated  lose  water  and  become  semi-vitri- 
fied. The  use  of  baked  clay  was  proposed  by  Wedgewood, 
and  the  apparatus  has  been  called  Wedgewood's  Pyrometer, 
or  fire  measure.  All  contrivances  for  measuring  heat  may  be 
graduated  so  as  to  correspond  with  the  scale  adopted  for  the 
mercurial  thermometer. 

It  is  most  interesting,  while  considering  the  vast  number 


TABLE  OP  TEMPERATURE.  87 

and  importance  of  the  phenomena  produced  by  heat,  to  ob- 
serve the  degrees  in  the  general  scale  of  temperature  at  which 
they  take  place.  In  the  following  table  a  selection  of  the  facts 
are  classified,  the  temperatures  being  all  referred  to  the  scale  of 
Fahrenheit's  thermometer. 

Table,  of  facts  connected  with  the  influence  of  heat  corres- 
ponding to  certain  temperatures. 

Fahrenheit.  Weclgewood. 

Highest  temperature  measured  -         -  32,277°  ...240° 

Chinese  porcelain  softened     -  -21,357  ...156 

Cast  iron  thoroughly  melted  -20,577  ...150 

Greatest  heat  of  a  common  smith's  forge  17,327  ...125 

Flint  glass  furnace  -  15,897  ...114 

Stone  ware  baked  in      -  -14,337  ...102 

Welding  heat  of  iron    -  -  13,427  ...    90  to  95 

Delft  ware  baked  in  -     6,407  ...  41 

Fine  gold  melts     -         -  -     5,237  ...   32 

Settling  heat  of  flint  glass      -  4,847  ...   29 

Fine  silver  melts  -     4,717  ...   28 

Brass  melts  -     3,807  ...   21 
Full  red  heat  (the  beginning  of  Wedge- 

wood's  Pyrometer)  -  -     1,077  ...     0 

Heat  of  a  common  fire  790 

Iron  red  in  the  dark      -  750 

Quicksilver  boils  -  660 

Linseed  oil  boils  -  600 

Fahrenheit. 

Lead  melts  -  -  594° 

Sulphur  melts        -  -  226 

Water  boils  -  212 

A  compound  of  three  parts  of  tin,  five  of  lead,  and 

eight  of  bismuth  melts  -  210 
Alcohol  boils  -  174 
Bees'-wax  melts  -  -  142 
Ether  boils  -  -  98 
The  present  medium  temperature  of  the  globe  -  50 
Ice  melts 32 


88  HEAT. 

Fahrenheit. 

Milk  freezes  -  -  30° 

Vinegar  freezes     --  ...  23 

Strong  wine  freezes       -  -  -  20 

Weak  brine  freezes        -  -  zero  0 

Quicksilver  freezes        -  below  zero  40 

Natural  temperature  observed  at  Hudson's  Bay       -  50 

Greatest  artificial  cold  yet  measured        -  -  91 

There  is  reason  for  thinking  that  the  higher  temperatures 
noted  in  this  table  appear  considerably  too  high,  owing  to  the 
insufficiency  of  the  thermometer  or  pyrometer  (Wedgewood's) 
by  which  they  were  estimated. 

It  is  a  curious  inquiry,  suggested  by  contemplating  the  pre- 
ceding table,  how  much  heat  may  yet  remain  in  bodies  at  the 
lowest  temperature  which  We  know?  No  conjecture  was  ha- 
zarded on  the  subject  until  Dr*  Irvine  thought  it  might  be  elu- 
cidated by  comparing  the  quantity  of  heat  which  becomes  la- 
tent in  a  body  on  changing  form,  with  the  capacity  of  the  body 
before  and  after  the  change.  For  instance,  with  respect  to  wa- 
ter, he  said,  as  it  requires  one-tenth  more  heat  to  make  a  cer- 
tain change  in  the  temperature  of  water  than  in  that  of  an  equal 
quantity  of  ice,  it  is  probable  that  ice-cold  water  just  contains 
altogether  one-tenth  more  heat  than  an  equal  quantity  of  ice 
at  the  melting  point:  then  as  we  know  the  water  to  contain 
exactly  140°  more  heat  than  the  ice,  viz.  its  latent  heat,  the 
whole  or  absolute  quantity  of  heat  in  it  must  be  ten  times  140°, 
or  1,400°.  By  applying  this  reasoning,  however,  to  other  sub- 
stances than  water,  it  is  proved  evidently  to  be  fallacious;  and 
the  conclusion  follows,  that  we  have  as  yet  no  means  of  solving 
the  question; — the  thermometer  no  more  telling  us  the  abso- 
lute quantity  of  heat  in  any  body  than  the  rising  and  falling  of 
the  water-surface  in  a  well,  tells  the  total  depth  of  the  well. 

From  what  is  said  in  the  last,  and  in  preceding  paragraphs, 
it  is  evident  that  the  thermometer  gives  very  limited  informa- 
tion with  respect  to  heat:  it  merely  indicates,  in  fact,  what 
may  be  called  the  tension  of  heat  in  bodies,  or  the  strength  of 
its  tendency  to  spread  from  them.  Thus  i't  does  not  discover 
that  a  pound  of  water  takes  thirty  times  as  much  heat  to  raise 


INFLUENCE  ON  CHEMICAL  UNIONS.  89 

its  temperature  one  degree,  as  a  pound  of  mercury;  nor  does 
it  discover  the  caloric  of  fluidity  absorbed  when  bodies  change 
their  form,  and  which,  indeed,  is  called  latent  heat,  only  be- 
cause hidden  from  the  thermometer;  nor  does  it  tell  that  there 
is  more  heat  in  a  gallon  of  water  than  in  a  pint;  and  if  an  ob- 
server did  not  make  allowance  for  the  increasing  rate  of  ex- 
pansion in  the  substance  used  as  a  thermometer,  as  the  tempe- 
rature increases,  he  would  believe  the  increase  of  heat  to  be 
greater  than  it  is;  and,  lastly,  when  a  fluid  is  used  as  a  ther- 
mometer, the  expansion  observed  is  only  the  excess  of  the  ex- 
pansion in  the  fluid  over  that  in  the  containing,  solid,  and  sub- 
ject to  all  the  irregularities  of  expansion  in  both  substances: — 
all  proving  that  the  indications  of  the  thermometer,  unless  in- 
terpreted by  our  knowledge  of  the  general  laws  of  heat,  no 
more  disclose  the  true  relations  of  heat  to  bodies,  than  the  mo- 
ney accidentally  in  a  man's  pocket  tells  his  rank  and  riches. 

"  Heat  by  its  different  relation  to,  different  substances  has 
a  powerful  influence  on  their  chemical  combinations." 
(See  the  Analysis,  page  13.) 

By  observations  made  and  recorded  through  by-gone  ages, 
man  has  now  come  to  know  that  the  substances  constituting  the 
world  around  him,  although  appearing  to  differ  in  their  nature 
almost  to  infinity,  are  yet  all  made  up  of  a  few  simple  elements 
variously  combined;  and  he  has  discovered  that  the  peculiar 
relations  of  these  elements  to  heat, — as  their  being  unequally 
expanded  by  it,  and  their  undergoing  fusion  and  vaporization 
at  different  temperatures,  furnish  him  with  ready  means  of  se- 
parating, combining,  and  new-modifying  them  to  serve  to  him 
most  useful  purposes.  Where  the  primitive  savage,  looking 
around  on  rocks  and  soils,  saw  in  their  diversified  aspect  almost 
as  little  meaning  as  did  the  inferior  animals  which  participated 
with  him  the  shelter  of  the  wood  or  cave;  his  son,  with  pene- 
tration sharpened  by  science,  descries  at  once  the  treasures  of 
the  mine,  and  aided  by  heat,  whose  wonderful  energies  he  has 
learned  to  control,  pursues  through  all  the  Protean  disguises 
of  ores  and  salts  and  solutions,  each  of  the  wished-for  sub- 
stances, until  he  secures  it  apart.  For  instance,  in  what  to  his 

12 


90  ILEAT. 

forefathers  for  thousands  of  years  appeared  but  a  red  dross,  he 
knows  that  there  lies  concealed  the  precious  iron—- king  of  me- 
tals! and  soon  forcing  this  in  his  ardent  furnace  to  assume  its 
metallic  form,  with  implements  made  of  it  he  afterwards  moulds 
all  other  bodies  to  his  will:  the  trees  from  the  forest  and  the 
rocks  from  the  quarry,  in  obedience  to  these,  come  to  be  fa- 
shioned by  him  as  if  they  were  of  soft  clay,  and  at  his  com- 
mand rise  into  the  magnificent  structures  of  palaces  and  ships, 
with  which  the  earth  is  now  beautified,  and  the  ocean  so  thick- 
ly covered.  The  minute  detail  of  the  relations  to  heat  of  par- 
ticular substances  forms  a  great  part  of  the  department  of  sci- 
ence called  chemistry^  (a  name  taken  from  an  Arabic  word 
signifying  fire;)  but  a  general  review  of  the  subject  belongs 
to  this  work. 

The  most  common  ores  of  metals  are  combinations  of  them 
Avith  oxygen,  carbonic  acid,  or  sulphur,  substances  all  of  which 
are  volatilized  at  much  lower  temperatures  than  the  metals. 
Now,  simple  roasting,  as  it  is  called,  or  strongly  heating  the 
ores,  suffices  often  to  drive  away  great  part  of  these  adjuncts^ 
and  where  additional  assistance  is  required,  it  is  obtained  by 
mixing  with  the  ore  something  which  when  heated  attracts  the 
substance  to  be  expelled  more  strongly  than  the  metal  does. 
Charcoal,  for  instance,  heated  with  an  oxid  ore,  takes  the  oxy- 
gen, and  flying  off  with  it  as  carbonic  acid,  leaves  at  the  bottom 
of  the  furnace  or  crucible  the  vivified  metal. 

Mercury  mixed  with  the  dross  of  a  mine,  dissolves  any  par- 
ticles of  gold  or  of  silver  existing  in  it,  and  the  ingredients  of 
the  solution  may  afterwards  be  obtained  separate  by  mere  heat- 
ing— the  mercury  passing  away  as  vapour  to  where  it  is  cooled 
and  again  condensed  for  repeated  use,  and  the  more  fixed  gold 
or  silver  remaining  pure  in  its  place, — -just  as  in  all  other  dis- 
tillations, as  that  of  spirit  from  wine,  or  of  essential  oils  from 
water,  &c.,  there  is  the  separation  by  heat  of  a  more  volatile 
from  a  less  volatile  substance.  The  only  difference  between 
what  is  called  drying  by  heat  and  distilling  is,  that  in  the  one 
case  the  substance  vaporized,  being  of  no  use,  is  allowed  to  es- 
cape or  be  dissipated  in  the  atmosphere;  while  in  the  other,  be- 
ing the  precious  part,  it  is  caught  and  condensed  into  the  liquid 
form, 


INFLUENCE  ON  CHEMICAL  UNIONS.  Hi 

A  piece  of  cold  charcoal  lies  in  the  air  for  any  length  of  time 
without  change:  but  if  heated  to  a  certain  degree,  the  mutual 
cohesion  of  its  particles  is  so  weakened,  that  is,  the  particles  are 
so  repelled  and  separated  from  each  other,  that  their  attraction 
for  the  oxygen  in  the  air  around  is  allowed  to  operate,  and  they 
combine  with  that  oxygen,  so  as  to  produce  the  phenomenon  of 
combustion.  The  same  is  true,  under  similar  circumstances, 
of  almost  any  dry  vegetable  or  animal  substance,  and  of  several 
of  the  metals. 

Nitre,  sulphur,  and  charcoal,  while  cold,  may  be  mixed  toge- 
ther most  intimately  without  any  change  taking  place;  but  if  the 
mixture,  or  any  part  of  it,  be  heated  to  a  certain  degree,  the 
whole  explodes  with  extreme  violence;  for  it  is  gunpowder. 
By  the  change  of  temperature,  and  the  consequently  altered  re- 
lative attractions  of  the  different  substances,  a  new  chemical  ar- 
rangement of  them  then  takes  place  with  the  intense  combustion 
and  expansion,  which  constitute  the  explosion. 

Sea  sand  and  soda  may  be  mixed,  and  even  ground  together, 
as  completely  as  possible;  but  if  they  remain  cold,  they  remain 
also  merely  an  opaque  and  useless  powder:  on  heating  the  mix- 
ture, however,  to  diminish  the  cohesion  of  the  particles  of  each 
substance  to  those  of  its  own  kind,  so  that  the  mutual  attractions 
of  the  two  substances  may  come  into  play,  they  melt  altogether, 
and  unite  chemically  into  the  beautiful  compound  called  glass; 
a  product,  than  which  art  has  formed  none  more  admirable — 
which,  in  domestic  use  for  instance,  is  fashioned  into  the  bril- 
liant chandelier  and  lustre,  into  the  sparkling  furniture  of  the 
side  board,  into  the  magnificent  mirror  plate,  and  which,  ex- 
tended across  our  window  openings,  admits  the  light  while  it 
repels  the  storm: 

Perhaps  the  influence  of  temperature  on  chemical  union  is 
nowhere  more  remarkably  exhibited,  than  in  retarding  or  has- 
tening the  decompositions  of  dead  vegetable  and  animal  sub- 
stances. The  functions  of  life  bring  into  combination,  to  form 
the  various  textures  of  organic  or  living  bodies,  chiefly  four 
substances,  viz.  carbon  or  coal;  the  ingredients  of  water,  or 
oxygen  and  hydrogen;  and  lastly,  nitrogen— which  substances, 
when  in  the  proportions  found  in  such  bodies,  have  but  slight 


1*2  HEAT. 

attraction  for  each  other,  and  all  of  which,  except  the  carbon? 
usually  exist  as  airs.  Their  connexion,  therefore,  is  easily  sub- 
verted; and  particularly  by  a  slight  change  of  temperature,  which 
either  so  weakens  their  mutual  hold  as  to  allow  new  arrangements 
to  be  formed,  or  altogether  disengages  the  more  volatile  of  them. 
At  a  certain  temperature,  a  solution  of  sugar  (which  consists  of 
the  three  substances  first  mentioned,  carbon,  oxygen,  and  hy- 
drogen,) undergoes  a  change  into  a  spirituous  wash,  from  which 
spirit  or  alcohol  may  then  be  obtained  by  distillation:  but  if  the 
heat  be  continued  under  certain  circumstances,  the  liquid  under- 
goes a  second  change,  or  new  arrangement  of  constituent  parti- 
cles, and  becomes  vinegar:  under  still  other  circumstances  it  un- 
dergoes a  third  change,  which  is  a  destructive  decomposition, 
or  rotting,  as  we  call  it,  and  the  oxygen  and  hydrogen  ascend 
away  as  airs.  But  sugar,  and  many  similar  vegetable  compounds, 
preserved  at  a  low  temperature,  remain  unchanged  for  ages. 

Again,  as  regards  dead  animal  substances,  we  find,  that  al- 
though at  a  certain,  not  very  elevated,  temperature,  they  un- 
dergo that  change  in  the  relations  of  their  elements  which  we 
call  putrefaction,  when  nearly  their  whole  substance  rises  again 
to  form  part  of  the  atmosphere,  still  at  or  below  the  temperature 
of  freezing  water,  they  remain  unaltered  for  any  length  of  time. 
In  the  middle  of  summer,  recently  caught  salmon,  or  other  fish, 
packed  in  boxes  with  ice,  is  conveyed  fresh  from  the  most  re- 
mote parts  of  Britain  to  the  capital.  In  our  warmest  weather, 
any  meat  or  game  may  be  long  preserved  in  an  ice  house.  In 
Russia,  Canada,  and  other  northern  countries,  on  the  setting  in 
of  the  hard  frosts,  when  the  inferior  animals  have  difficulty  in 
finding  food,  the  inhabitants  kill  their  winter  supply,  and  store 
their  provender  of  frozen  flesh  or  fowl,  as  in  other  countries 
men  store  that  which  is  salted  or  pickled.  But  the  most  striking 
instance  of  this  kind  we  can  adduce  is  the  .fact,  that  on  the  shore 
of  Siberia,  in  1  SOI,  in  a  vast  block  or  island  of  ice,  of  which 
the  surface  was  then  more  melted  than  in  the  preceding  sum- 
mers, the  carcase  of  an  antediluvian  elephant  was  found,  per- 
fectly preserved — an  elephant  differing  materially  from  those 
now  existing  on  earth,  but  its  skeleton  exactly  correspond- 
ing with  the  specimens  found  deep  buried  in  various  countries, 


INFLUENCE  ON  ANIMATED  BEING*.  93 

The  creature  was  soon  discovered  by  the  hungry  bears  of  the 
district,  which  were  seen  tearing  off  its  hairy  side,  and  feeding 
on  its  flesh,  as  fresh  almost  as  if  it  had  lived  yesterday,  although 
it  must  have  been  of  an  era  long  anterior  to  that  of  any  existing 
monument  on  earth,  of  human  art,  or  even  of  human  being. 
Long  after  it  fell  from  the  ice  to  the  sandy  beach,  and  when  its 
tusks  had  been  carried  away  for  sale  by  a  Tungusian  fisherman, 
and  its  flesh  had  been  nearly  devoured,  a  naturalist  who  visited 
it  found  an  ear  still  perfect,  and  its  long  mane,  and  part  of  its 
upper  lip,  and  eye  with  the  pupil  yet  distinguishable,  which 
had  opened  on  the  glories  of  a  former  or  younger  world! 
About  30  Ibs.  weight  of  its  hair,  which  had  been  trodden  into 
the  sand  by  the  bears  while  eating  the  carcase,  was  collected, 
and  is  now  preserved  in  different  museums  of  natural  curiosities; 
some,  for  instance,  in  the  museum  of  the  London  College  of  Sur- 
geons. 

"  Heat  has  a  powerful  influence  also  on  animated  nature^ 
both  vegetable  and  animal.79  (Read  the  Analysis,  page  13. 

As  the  detail  of  the  relations  of  heat  to  particular  inanimate 
substances  belongs  to  the  province  of  chemistry,  so  does  the 
detail  of  its  relations  to  particular  living  vegetables  and  animals 
belong  to  the  department  of  Physiology;  but  a  general  re- 
view of  the  subject  is  required  in  a  treatise  on  Natural  Philo- 
sophy. 

The  influence  which  heat,  exerts  on  inanimate  nature,  is 
more  immediately  and  completely  perceived  by  the  common 
mind,  than  its  influence  on  beings  which  have  life.  Thus  to 
all  it  is  obvious,  that  the  contrast  between  a  winter  and  summer 
landscape,  is  owing  chiefly  to  the  effect  of  heat  on  the  water  of 
the  landscape; — that  during  its  absence  in  winter,  there  is  the 
dry  barren  deformity  of  accumulated  ice  and  snow,  covering 
every  thing,  the  roads  impassable,  the  rivers  bound  up,  per- 
haps hidden,  the  air  deprived  of  moisture,  and  loaded  often 
with  powdery  drift; — and  that  when  warmth  comes,  the  living 
streams  again  appear,  gliding  their  way,  the  cascades  pour,  the 
rills  murmur,  the  canals  once  more  offer  their  bosoms  to  the 
boats  of  commerce,  the  lake  and  pool  again  show  their  level 


94  HEAT. 

face,  reflecting  the  glories  of  the  heavens,  and  the  genial  shower 
falls  upon  the  bosom  of  the  softened  earth,  become  ready  to  re- 
ceive the  spade  or  the  ploughshare.  Now  this  change  is  not 
at  all  greater  than  what  happens  to  a  winter  tree  acted  upon  by 
the  warmth  of  spring. — To  take  another  instance  from  inani- 
mate nature,  it  may  be  said  with  truth,  that  heat  applied  to  the 
cold  boiler  of  a  steam-engine,  is  the  cause  of  all  its  succeeding 
motions;  of  the  heaving  of  its  beam  and  pumps,  the  opening 
and  shutting  of  its  valves,  the  turning  of  its  wheels,  and  its 
ultimate  performance  of  work,  as  of  spinning,  or  weaving,  or 
grinding,  or  propelling  vehicles  by  land  and  water;  but  as  truly 
may  it  be  said,  that  heat  coming  to  a  seed  which  has  lain  cold 
for  ages,  is  the  cause  of  its  immediate  germination  and  growth; 
or  coming  to  a  lately  frozen  tree  is  the  cause  of  the  rising  of 
its  sap,  the  new  budding  and  unfolding  of  its  leaves  and  blos- 
soms, the  ripening  of  its  fruit.  And  what  is  true  of  one  seed 
or  tree,  is  true  of  the  whole  of  the  vegetable  creation.  When 
the  warm  gales  of  spring  have  once  breathed  on  the  earth,  it 
soon  becomes  covered,  in  field  and  in  forest,  with  its  thick  garb 
of  green,  and  soon  opening  flowers  or  blossoms  every  where 
breathe  back  again  a  fragance  to  heaven.  Among  these  the  he- 
liotrope is  seen  always  turning  its  beautiful  disc  to  the  sun, 
and  many  delicate  flowers  only  open  their  leaves  to  catch  the 
direct  solar  ray,  but  close  them  often  even  when  a  cloud  inter- 
venes, and  certainly  when  the  chills  of  night  approach.  On 
the  sunny  side  of  a  hill,  or  in  the  sheltered  crevice  of  a  rock, 
or  on  a  garden  wall  with  warm  exposure,  there  may  be  pro- 
duced grapes,  peaches,  and  other  delicious  fruits,  which  will 
not  grow  in  situations  of  an  opposite  character — all  acknow- 
ledging heat  as  the  immediate  cause,  or  indispensable  condition, 
of  vegetable  life. 

But  among  animals,  too,  the  effects  of  heat  are  equally  re- 
markable. The  dread  silence  of  winter,  for  instance,  is  suc- 
ceeded in  spring  by  one  general  cry  of  joy.  Aloft  in  the  air 
the  lark  is  every  where  carolling;  and  in  the  woods  and  shrub- 
beries, a  thousand  little  throats  are  similarly  pouring  forth  their 
songs  of  gladness — during  the  day,  the  thrush  and  blackbird 
near  our  dwellings,  are  heard  above  the  rest,  and  with  the  even- 


INFLUENCE  ON  ANIMATED  BEINGS.  95 

ing  comes  the  sweet  nightingale; — for  all  of  which  it  is  the  sea- 
son of  love  and  of  exquisite  enjoyment.  And  it  is  equally  so 
for  animal  nature  generally:  in  favoured  England,  for  instance, 
in  April  and  May  the  whole  face  of  the  country  resounds  with 
lowings  and  bleatings  and  barkings  of  joy.  And  even  man,  the 
master  of  the  whole,  and  whose  mind  embraces  all  times  and 
place,  is  far  from  being  insensible  to  this  change  of  season.  His 
far-seeing  reason  of  course  draws  delight  from  the  anticipation 
of  autumn,  with  its  fruits;  and  his  benevolence  rejoices  in  the 
happiness  observed  among  all  inferior  creatures;  but  indepen- 
dently of  these  considerations,  on  his  own  frame  the  returning 
warmth  exerts  a  direct  influence.  In  early  life,  when  the  na- 
tural sensibilities  are  yet  fresh  and  unaltered  by  the  habits  of 
artificial  society,  spring  to  man  is  always  a  season  of  delight. 
The  eyes  brighten,  the  whole  countenance  is  animated,  and 
the  heart  feels  as  if  new  life  were  come,  and  has  longings  for 
fresh  objects  of  endearment.  Of  those  who  have  passed  their 
early  years  in  the  country,  or,  among  the  charms  of  nature,  as 
contrasted  with  the  arts  of  cities,  there  are  few  who,  in  their 
morning  walks  in  spring,  have  not  experienced  without  very 
definite  cause,  a  kind  of  tumultuous  joy,  of  which  the  natural 
expression  would  have  been,  how  good  the  God  of  nature  is 
to  us!  Spring  is  a  time  when  sleeping  sensibility  is  roused  to 
feel  that  there  lies  in  nature  more  than  the  grosser  sense  per- 
ceives. The  heart  is  then  thrilled  with  sudden  extacy,  and 
wakes  to  aspiration  of  sweet  acknowledgment. 

Besides  these  effects  of  heat,  which  are  comparatively  tran- 
sient as  being  connected  with  the  seasons,  there  are  other  ef- 
fects on  animated  nature  of  a  more  permanent  character.  Cer- 
tain species  of  vegetables  and  animals,  by  their  relation  to  heaty 
are  confined  to  certain  latitudes  or  climates;  as  the  orange  tree 
and  bird  of  paradise,  to  warm  regions;  the  fir  tree,  and  arctic 
bear,  to  those  that  are  colder; — and  when  individuals  of  either 
class  can  support  diversity  of  climate,  they  acquire  a  certain 
character  according  to  the  climate, — as  seen  in  the  sheep  and  dogs 
of  the  various  regions  of  the  earth.  In  this  latter  respect  there 
is  no  instance  more  interesting  than  that  furnished  by  the  va- 
rieties of  the  human  race.  Assuming  that  the  whole  sprung: 


U*j  HEAT. 

from  one  stock,  what  a  contrast  is  there  between  the  native  of 
Central  Africa,  of  temperate  Europe,  and  of  the  Polar  Zone: 
between  the  Negro,  the  Greek,  and  the  Esquimaux:  or  again, 
between  the  dark  slender  children  of  Hindostan,  the  strongly- 
knit  active  Roman  or  Spaniard,  and  the  taller,  ruddy,  powerful 
Briton.  And  in  the  female  sex  of  the  last-named  countries,  we 
may  remark  the  gentleness  and  singular  devotedness  of  fche  In- 
dian woman,  the  more  commanding  dark  eye  and  gesture  of 
the  graceful  nymph  of  Italy  or  Spain,  and  the  happily  attempered 
mixture  of  these  qualities  in  the  fair  and  much-favoured  daugh- 
ters of  Britain. 

The  very  important  influence  of  heat  upon  the  temporary 
bodily  state  of  animals,  becomes  an  object  of  much  study  to  the 
physician:  it  explains,  among  many  other  facts,  the  connexion 
of  temperature  with  the  rise  of  fevers  and  other  pestilences,  the 
powerful  remedial  efficacy  of  hot  and  cold  bathing,  of  changes 
of  climate,  of  regulating  the  temperature  of  air  breathed  by  in- 
valids, the  protection  from  clothes,  houses,  &c. 

"  The  great  natural  source  of  heat  is  the  sun.-'     (See  the 
Analysis,  page  13.) 

To  be  assured  of  this,  it  is  only  necessary  to  think  of  the  com- 
parative temperatures  of  night  and  day,  of  climates  and  of  sea- 
sons, and  to  reflect  that  the  sun  is  the  sole  cause  of  the  differ- 
ences. We  need  not  wonder,  then,  that,  to  many  savage  nations 
seeking  the  source  of  their  life  and  happiness,  the  sun  has  been 
the  object,  not  only  of  admiration,  but  of  worship. 

The  heat  comes  from  the  sun  with  his  light.  If  a  sun-beam 
enter  by  a  small  opening  an  apartment  otherwise  closed  and 
dark,  it  illuminates  intensely  the  spot  or  object  on  which  it  first 
falls,  and  its  light  being  then  scattered  around,  all  the  objects  in 
the  room"  become  feebly  visible.  Again,  a  cold  thermometer, 
held  to  receive  the  direct  ray,  rises  much;  while  in  any  other  si- 
tuation it  is  less  affected:  proving  the  heat  to  be  like  the  light, 
widely  diffused,  and  so  to  lose  proportionately  of  its  intensity. 
Light  passes  from  the  sun  to  the  earth  in  about  eight  minutes  of 
time,  as  will  be  fully  .explained  in  a  future  chapter;  and  there  is 
every  reason  to  conclude  that  heat  travels  at  the  same  rate, 


THE  SUN  THE  GREAT  SOURCE.  97 

Human  art  can  gather  the  sun-beams  together,  and  by  the 
intense  heat  produced  in  the  focus  of  their  meeting,  furnishes 
another  proof  that  the  sun  is  the  great  source  of  heat.  A  pane 
of  glass  in  a  window,  or  a  small  mirror,  will  reflect  the  sun's 
ray  so  as  to  offend  an  eye  receiving  it  at  a  distance  of  miles — as 
may  be  observed  soon  after  the  rising,  or  before  the  setting  of 
the  sun,  when  his  ray  is  nearly  horizontal, — and  the  heat  ac- 
companies the  ray;  for  by  many  such  mirrors  directed  towards 
one  point,  a  combustible  object  placed  there  would  be  inflamed. 
Archimedes  set  fire  to  the  Roman  ships  by  sun-beams,  returned 
from  many  points  to  one,  his  god-like  genius  thus  rivalling  by 
natural  means,  the  supposed  feats  of  fabled  Jupiter  with  his 
thunderbolts.  Again,  when  the  light  of  a  broad  sun-beam  is 
made  by  a  convex  glass  or  lens  to  converge  to  one  point  or  fo- 
cus, the  concentrated  heat  is  also  there — for  a  piece  of  metal 
held  in  the  focus  drops  like  melting  wax:  and  if  the  glass  be 
purposely  moved,  its  focus  will  pierce  through  the  most  obdu- 
rate substances,  as  red  hot  wire  pierces  through  paper  or  wood. 
A  hunter  on  his  hill,  and  travelling  hordes  on  the  plains,  often 
conveniently  light  their  fires  at  the  sun  himself,  by  directing  his 
energies  through  a  burning-glass. 

The  direct  ray  of  the  sun,  simply  received  into  a  box  which 
is  covered  with  glass  to  exclude  the  cold  air,  and  is  lined  with 
charcoal  or  burned  cork  to  absorb  heat,  and  to  prevent  the  es- 
cape of  heat  once  received,  will  raise  a  thermometer  in  the  box 
to  the  temperature  of  230°  degrees  of  Fahrenheit,  a  temperature 
considerably  above  that  of  boiling  water.  And  the  experiment 
succeeds  in  any  part  of  the  earth  where  there  is  a  clear  atmos- 
phere, and  where  the  sun  attains  considerable  apparent  altitude. 
We  see  therefore  that  a  solar  oven  might  in  some  cases  be  used. 
In  operating  with  the  apparatus  suggested  by  the  author,  and 
described  at  page  75,  for  distilling  water  by  the  heat  of  the  sun, 
the  vessel  intended  to  absorb  the  heat,  and  to  act  as  the  still, 
should  be  enclosed  in  a  case  lined  and  covered  as  above  de- 
scribed. 

Reflecting  on  such  facts  as  now  recorded,  and  on  the  globu- 
lar form  and  the  motions  of  our  earth,  we  have  a  measure  of 
the  differences  of  climate  and  of  season  that  should  be  found 

13 


98  HEAT. 

upon  the  earth.  It  is  evident  that  the  part  of  the  globe  turned 
directly  to  the  sun,  receives  his  rays  as  abundantly  as  if  it  were 
a  perfect  plane,  similarly  facing  him,  while  on  parts,  which,  as 
viewed  from  the  sun,  would  be  called  the  sides  of  the  globe, 
with  the  increasing  obliquity  of  aspect,  an  equal  breadth  or 
quantity  of  rays  is  spread  over  a  larger  and  a  larger  surface; 
and  at  the  very  edge  the  light  passes  level  with  the  surface, 
and  altogether  without  touching.  The  sunny  side  of  many  a 
steep  hill,  in  England,  receives  the  sun's  rays  in  summer  as 
perpendicularly  as  the  plains  about  the  equator;  and  such  hill- 
side is  not  heated  like  these  plains,  only  because  the  air  over 
it  is  colder — just  as  mountain-tops,  even  at  the  equator,  owing 
to  the  rarified  and  therefore  cold  air  around  them,  remain  for 
ever  hooded  in  snow.  In  England,  at  the  time  of  the  equi- 
noxes, a  level  plain  receives  only  about  half  as  much  of  the 
sun's  light  and  heat  as  an  equal  extent  of  level  surface  near  the 
equator;  and  in  the  short  days  of  winter  it  receives  conside- 
rably less  than  a  third  of  its  summer  allowance. 

There  are  few  contrasts  in  nature  more  striking  than  some 
of  the  consequences  of  different  intensity  of  the  sun's  influ- 
ence:— that,  for  instance,  of  the  inhabitants  of  India,  at  mid- 
day, in  the  hot  season,  with  the  thermometer  at  120°,  running  to 
the  shade  of  their  bungalows,  darkening  their  windows,  hanging 
wetted  mats  upon  the  walls  and  roofs,  and  sprinkling  the  floors, 
fanning  themselves  with  ever-moving  punkas,  and  feeling  the 
slightest  covering  or  exertion  too  much — while,  on  the  other 
hand,  the  dwellers  in  Greenland,  with  the  thermometer  below 
zero,  are  loaded  with  furs,  and  are  seeking  the  direct  sunshine 
or  heat  from  a  fire,  as  their  life  and  comfort.  Again,  there  is 
the  contrast  observed  on  passing,  as  the  author  once  did,  in  ten 
days,  from  such  a  paradise  as  Rio  de  Janeiro,  with  all  its  ve- 
getable riches,  to  Tristan  da  Cunha,  and  the  Isle  of  Desolation 
in  the  Southern  Ocean,  which  exhibit  only  cold  and  naked 
rocks;  but  yet  where  the  scene  was  swarming  with  its  appro- 
priate inhabitants — the  sea  with  seals,  and  the  air  with  clouds 
of  sea  fowl,  playing  over  the  never-resting  waves  like  flakes 
of  eddying  snow.  Were  a  person  for  a  moment  to  doubt  whe- 
ther the  sun  be  the  real  cause  of  such  differences,  and  of  cer- 


THE  SUN  THE  GREAT  SOURCE.  99 

tain  creatures  being  found  only  in  certain  zones  of  the  earth, 
let  him  reflect  on  the  extraordinary  migration  of  animals,  which 
have  their  home  not  in  any  fixed  region,  hut  wherever  the  sun 
has  for  a  time  a  particular  degree  of  influence,  and  which  ac- 
cordingly follow  the  sun  in  the  changes  of  season.  We  have 
the  swallow  in  such  numbers,  coming  to  visit  the  British  isles 
in  the  spring,  to  play  over  our  woods  and  waters,  in  pursuit  of 
the  insects  which  the  heat  then  breeds  in  the  air, — welcome 
harbingers  of  the  coming  summer  and  its  riches;  and  in  au- 
tumn the  same  creatures  are  seen  congregating  on  our  shores, 
to  wing  their  flight  back  in  united  multitudes  to  more  southern 
countries,  where,  in  turn,  there  is  a  temperate  influence  of  the 
sun.  The  same  season  brings  to  England  the  nightingale,  and 
makes  our  woodlands  resound  with  the  note  of  the  cuckoo.  In 
the  waters  of  our  bays  and  coasts,  again,  there  appear  with  the 
seasons  the  vast  shoals  of  fish,  as  the  herring  and  mackarel, 
which  prove  such  abundant  food  for  millions  of  human  beings; 
and  the  salmon,  at  stated  times,  penetrates  from  the  ocean  far 
up  the  mountain-streams,  to  deposite  its  spawn  for  future  sup- 
ply;— all,  by  their  movements,  contributing  to  the  harmonious 
and  beneficent  system  of  the  universe. 

With  respect  to  the  sun  as  a  source  of  heat,  there  have  been 
two  opinions  among  philosophers;  one  class  believing  that  the 
sun  is  an  intensely  heated  mass,  which  radiates  its  heat  and 
light  around,  like  a  mass  of  intensely  heated  iron:  and  another 
class  holding  that  heat  is  merely  an  affection  or  state  of  an 
ethereal  fluid,  which  occupies  all  space,  as  sound  is  an  affection 
or  motion  of  air,  and  that  the  sun  may  produce  the  phenomena 
of  light  and  heat  without  waste  of  its  temperature  or  substance, 
as  a  bell  may  without  waste  continue  to  produce  sound:  hold- 
ing farther,  that  the  sun  below  its  luminous  atmosphere  may 
be  habitable  even  by  such  animals  as  live  on  this  earth.  Those 
who  take  the  first  view,  are  awakened  to  the  dread  contempla- 
tion of  a  universe  carrying  in  itself,  if  its  laws  remain  constant, 
the  seeds  of  its  certain  decay,  or  at  least  of  great  periodical  re- 
volutions: the  others  may  view  the  universe  as  destined  to  last 
nearly  unchanged,  until  a  new  act  of  the  will  of  its  Creator 
shall  again  alter  or  destroy  it. 


100  HEAT. 

Of  one  fact  there  can  be  no  doubt,  viz.  that  the  present  tem- 
perature of  the  earth  is  much  lower  than  the  temperature  in 
remote  past  time.  The  rocks,  called  primitive,  as  granite  and 
gneiss,  constituting  the  interiors  of  our  great  mountain  masses, 
and  the  substrata  of  our  plains,  bear  evident  marks  of  having 
been  at  one  period  in  a  molten  state,  from  which  they  have 
been  solidified  by  a  very  gradual  cooling;  and  even  the  whole 
mass  of  the  earth  at  some  time  must  have  been  so  fluid  or  soft, 
as,  in  obedience  to  gravity,  to  have  assumed  its  rounded  form, 
and  in  obedience  to  the  centrifugal  force  of  its  whirling,  to 
have  bulged  out,  at  its  great  circumference  or  equator,  the  se- 
venteen miles  which  its  equatorial  diameter  exceeds  the  polar; 
the  same,  by  the  by,  in  degrees  corresponding  to  the  various 
speed  of  rotation,  being  true  of  all  the  other  planets  belonging 
to  the  solar  system.  Again,  while  in  excavating  below  the 
surface  of  the  globe,  or  in  examining  its  structure  as  exposed 
to  view  by  volcanic  or  other  convulsions,  men  encounter  in 
very  many  situations  a  thickness  of  more  than  a  mile,  of  the 
wreck  and  remains  of  former  states  of  the  world — as  on  dig- 
ging eighty  feet  under  vineyards  near  Mount  Vesuvius,  they 
encounter  the  buried  cities  of  Herculaneum  and  Pompeii — they 
farther  discover  that  the  animal  and  vegetable  remains  buried, 
without  number,  in  the  present  cold  climates  of  the  earth,  and 
evidently  resting  near  where  the  creatures  lived,  are  all  of  kinds 
now  inhabiting  only  the  warmer  or  tropical  regions.  Lastly, 
in  the  operations  of  mining,  the  deeper  men  go,  the  higher 
they  find  the  temperature  to  be,  at  the  rate  of  a  degree  for 
about  200  feet  of  descent;  which  fact,  as  heat  tends  to  equable 
diffusion,  proves  both  that  the  central  heat  of  our  earth  must 
have  had  another  source  than  a  radiation  from  the  sun  of  the 
present  intensity;  and  that  the  surface  of  the  earth  is  now  ra- 
diating away  more  heat  than  it  receives  from  the  sun.  The 
conclusion  then  follows,  that  the  temperature  of  the  world  is 
still  falling,  although,  perhaps,  so  slowly  that  a  change  may 
not  be  detected  even  within  centuries.  Possibly  in  very  re- 
mote antiquity  that  may  have  been  true  which  the  early  Greeks 
erroneously  thought  true  in  their  day,  viz.  that  the  equator  of 


COMBUSTION.  101 

the  earth,  by  reason  of  its  great  heat,  was  a  barrier  impassable 
by  man  between  the  northern  and  southern  hemispheres. 

"  Electricity  a  source  of  heat."     (See  the  Analysis.) 

This  subject  can  only  be  satisfactorily  entered  upon  in  the 
chapter  devoted  exclusively  to  electricity,  and  is  therefore  de- 
ferred. Suffice  it  here  to  say,  that  while  an  electrical  discharge 
or  current  passes  from  one  situation  to  another,  the  substance 
serving  as  a  conductor  is  often  heated,  melted,  or  dissipated,  in 
such  a  manner  as  to  make  it  doubtful  whether  we  possess  any 
more  powerful  means  of  producing  these  effects.  We  may  re- 
mark, too,  that  in  certain  cases  of  the  electrical  current,  the 
heat  is  accompanied  by  as  intense  a  light  as  art  can  exhibit. 

"  Combustion  and  other  chemical  actions  as  sources  of 
heat."     (See  the  Analysis,  page  13.) 

Of  the  phenomena  of  nature  there  is  perhaps  none,  which, 
to  the  uninstructed,  appears  so  inexplicable  and  so  wonderful 
as  that  of  fire  or  combustion — whether  contemplated  in  its 
beauty  or  in  its  terrors.  Fire  is  seen  in  its  beauty  when  used 
by  man  for  his  domestic  purposes,  as  when  it  blazes  cheerfully 
over  his  parlour  hearth,  or  beams  around  its  steady  light  from 
his  lamps  and  chandeliers.  It  is  seen  again  in  its  terrors,  when 
spreading  by  accident  from  some  focus,  it  envelopes  in  sudden 
flame  the  draperies  and  other  furniture  of  an  apartment;  or 
when  breaking  from  a  first  apartment  it  rages  through  a  whole 
habitation,  consuming  as  its  food  and  carrying  in  its  long  flames 
to  the  sky,  almost  every  thing  save  the  stone  walls,  left  as  a 
blackened  skeleton;  or,  again,  when  still  wider  spread,  it  is  at 
the  same  dread  moment,  devouring  with  deafening  uproar  a 
whole  town  or  a  forest: — nay,  it  is  terrible  fire,  labouring  with- 
in the  bowels  of  the  earth,  which  first  prepares  and  then  urges 
up  to  heaven  the  volcanic  eruption  of  flame  and  red-hot  rocks, 
during  which  the  region  around  often  quakes  and  is  uptorn, 
with  demolition  of  its  cities  into  sudden  tombs  of  the  inhabi- 
tants, with  change  in  the  course  of  its  rivers,  with  conversion 
of  its  plains  into  lakes,  or  of  its  lake  beds  into  dry  land.  Fire 
appears  terrible  also  in  the  meteors  of  night;  and  worse  than 


102  HEAT. 

terrible  when,  intentionally  lighted  by  human  hands,  it  bursts 
from  the  cannon's  womb  to  produce  the  carnage  of  the  battle. 
Fire  among  many  nations  of  antiquity  was  regarded  with  awe 
and  holy  reverence,  the  sun  himself  being  honoured  chiefly  as 
its  concentration  or  supposed  abode.  Then  there  were  sacred 
fires  in  many  of  the  temples,  and  fire  was  used  to  complete  the 
splendour  of  the  most  august  ceremonies.  But,  more  remark- 
able still,  Moses,  a  worshipper  of  the  one  true  God,  has  re- 
corded of  the  Burning  Bush,  and  of  burnt  offerings  made  to 
that  God:  and  at  the  present  day,  in  many  Christian  churches, 
there  are  ever-burning  lamps  and  frequent  magnificent  illumi- 
nations. Now,  this  wondrous  principle  of  fire,  which  when 
the  savage  man  first  saw  it  spreading,  perhaps,  after  the  thun- 
der clap  or  the  rubbing  of  forest  branches  in  a  storm,  so  as  to 
threaten  universal  destruction,  he  so  naturally  accounted  the 
demon,  if  not  the  God  of  nature, — this  principle  man's  art  has 
now  tamed  to  be  a  most  obedient,  and  by  far  the  most  useful 
of  all  his  servants.  Fire,  being,  in  truth,  but  a  concentration 
of  the  element  of  heat,  which,  in  its  tranquil  and  invisible 
diffusion,  we  have  already  contemplated  as  the  beneficent  life 
or  soul  of  the  universe — the  cause  of  seasons  and  climates,  and 
of  all  the  changes  or  activity  which  distinguish  a  living  world 
from  a  dead  and  frozen  mass;  man,  by  acquiring  command 
over  it,  can  command  heat  when  and  where  he  wills,  and  thus 
truly  becomes  in  a  second  degree  the  ruler  of  nature.  Fire,  in 
man's  service,  may  be  figured  as  a  legion  of  spirits  to  whom 
no  labour  is  difficult,  and  who,  in  any  particular  case,  have 
power  or  magnitude  exactly  proportioned  to  the  quantity  of 
food  or  fuel  afforded;  of  whom,  moreover,  man  can  at  any  mo- 
ment conjure  up  one  or  many  by  the  magic  stroke  of  his  flint 
and  steel.  In  every  private  dwelling  he  has  of  these  fiery  spi- 
rits as  domestic  servants — in  the  kitchen  and  in  the  parlour. 
In  his  manufactories  they  are  melting  glass  for  him,  and  re- 
ducing ores,  and  boiling  and  evaporating  for  a  hundred  pur- 
poses. But  it  is  chiefly  while  chained  to  the  steam  engine, 
that  they  show  their  miraculous  powers: — as  when,  putting 
forth  a  giant's  strength,  they  heave  a  river  from  the  bottom  of 
a  mine,  or  urge  a  vast  ship  through  the  winter  storm ;  or  when 


COMBUSTIOIT.  103 

in  nice  dexterity  equalling,  if  not  surpassing,  what  human 
hands  can  effect,  they  twist  the  silken  or  cotton  threads,  and 
weave  them  into  most  delicate  fabrics.  Men,  now  grown  fa- 
miliar with  such  prodigies,  have  almost  ceased  to  be  moved  by 
them;  but  few  persons  can  resist  a  feeling  of  wonder  and  admi- 
ration when  chemistry,  in  its  progress  of  discovery,  every  now 
and  then  calls  forth  the  hidden  spirit  of  combustion  in  some 
new  or  less  familiar  guise: — for  instance,  when  a  piece  of  iron 
wire,  lighted  as  a  taper  in  oxygen  gas,  burns  with  such  resplen- 
dent brilliancy; — or  when  phosphorus  similarly  placed,  throws 
around  its  overpowering  flood  of  flame; — or  when  small  por- 
tions of  the  metal  called  potassium,  being  cast  upon  the  sur- 
face of  water,  become  as  beads  of  most  intense  light  running 
about  there,  and  crossing  as  in  a  merry  dance; — or,  lastly, 
when  flames  produced  from  particular  substances  are  seen  rising 
deep-tinged  with  most  vivid  and  beautiful  colours. 

Singularly  interesting  then,  to  philosophers,  as  in  such  par- 
ticulars the  phenomenon  of  combustion  must  always  have  ap- 
peared, one  may  wonder  that  its  true  nature  could  remain  to  them 
so  long  a  mystery;  but  until  the  admirable  researches  of  Davy, 
made  only  a  few  years  ago,  their  conjectures  had  scarcely  ap- 
proached the  truth.  An  opinion  long  prevailed,  that  in  every 
combustible  substance  there  was  present  a  certain  quantity  of 
a  something  denominated  phlogiston,  which,  on  being  disen- 
gaged or  separated,  became  obvious  to  human  sense  as  light  and 
heat.  The  white  oxid  of  zinc,  for  instance,  named  the  flowers 
of  zinc,  and  into  which  the  metal  is  changed  by  burning,  was 
supposed  to  be  the  metal  deprived  of  its  phlogiston;  and  when 
the  metal  again  appeared,  on  this  oxid  being  heated  with  char- 
coal, it  was  supposed  simply  to  have  recovered  phlogiston  from 
the  charcoal.  The  illustrious  Lavoisier  had  the  merit  of  mosi 
clearly  disproving  this  hypothesis,  by  showing,  for  instance,, 
that  the  flowers  of  zinc  were  heavier  than  the  piece  of-  metal 
from  which  they  were  produced,  by  the  exact  weight  of  the  ox- 
ygen gas,  which  disappeared  in  the  combustion,  &c. ;  and  he 
showed  further,  that  in  this  and  many  other  cases,  combustion 
was  merely  the  act  of  two  substances  combining  chemically;  but 
he  fell  into  an  error  almost  as  great  as  that  which  he  overthrew, 


104  HEAT. 

by  supposing  that  oxygen  had  always  to  be  one  of  the  com- 
bining substances,  and  that  the  heat  and  light  given  out  in  every 
case  had  been  previously  latent  in  that  oxygen. 

When  Sir  Humphrey  Davy  began  his  labours  on  the  subject, 
than  which  labours  there  is  not  perhaps  on  record  a  more  per- 
fect specimen  of  truly  scientific  research,  it  was  already  known 
that  bodies  when  compressed,  or  by  any  means  reduced  in  bulk, 
generally  gave  out  a  part  of  their  heat,  as — when  air  condensed 
in  the  match  syringe  lights  tinder, — or  when  water  and  sul- 
phuric acid,  uniting  into  a  compound  of  smaller  volume  than  the 
separate  ingredients,  become  very  hot, — or  when  water  poured 
upon  quicklime  to  slake  it,  and  becoming  solid  with  it,  produces 
heat  sufficient  to  inflame  wood,  as  has  been  fatally  proved  by  the 
burning  of  many  lime-loaded  ships; — it  being  evident,  moreover, 
that  the  heat  produced  during  the  chemical  unions  depended 
more  upon  the  energy  of  the  action  which  united  the  substances 
than  upon  the  change  of  volume  produced. 

Farther,  it  was  known  that  any  substance  having  its  tempera- 
ture raised,  by  whatever  means,  to  800°  or  more  of  Fahrenheit's 
thermometer,  became  incandescent  or  luminous, — as  when  iron, 
or  stone,  or  any  substance  not  dissipated  by  heat,  is  placed  in  a 
common  fire; — in  the  first  degree  the  substance  being  said  to  be 
red-hot,  and  at  higher  temperatures  to  be  white-hot. 

Now,  out  of  these  two  truths  Davy  constructed  his  explanation. 
He  asserted,  that  in  any  case,  combustion  is  merely  the  ap- 
pearance produced  when  substances,  which  have  perhaps  still 
stronger  attraction  for  each  other  than  quicklime  and  water,  are 
combining  chemically,  so  as  to  become  heated  at  least  to  the  de- 
gree of  incandescence.  During  the  phenomenon  there  is  not,  as 
was  formerly  supposed,  something  altogether  consumed  or  de- 
stroyed, or  something  called  phlogiston  escaping;  the  substances 
concerned  are  but  assuming  a  new  form  or  arrangement.  Thus 
if  a  piece  of  charcoal  be  enclosed  in  a  glass  vessel  filled  with  air, 
and  of  which  the  mouth  dips  into  a  liquid  to  confine  the  air,  and 
if  the  charcoal  be  then  heated  to  a  certain  degree,  by  means  of  a 
burning-glass  or  otherwise,  the  cohesion  of  its  particles  gives 
way  to  their  attraction  for  the  oxygen  of  the  air  around  them, 
and  they  immediately  begin  to  combine  with  the  air  so  energeti- 


COMBUSTION.  105 

cally  as  to  produce  a  heat  still  much  greater,  accompanied  by 
the  light  or  incandescence  of  combustion.  The  charcoal,  under 
these  circumstances,  soon  entirely  disappears,  or  is  dissolved  in 
the  air,  as  sugar  may  be  dissolved  in  water;  but  if  the  air  be  af- 
terwards weighed,  it  is  found  to  have  gained  in  its  weight  the 
exact  weight  of  the  charcoal  which  has  disappeared;  and  a  che- 
mist can  again  separate  the  charcoal  from  the  air,  and  use  either 
for  any  purpose  as  before.  In  like  manner,  if  a  piece  of  iron 
wire  be  heated  at  one  end,  which  is  then  plunged  into  a  jar  of 
oxygen  gas,  it  will  burn  as  a  most  brilliant  taper,  and  will  gra- 
dually fall  in  the  form  of  oxidized  drops,  or  scales  of  iron,  to 
the  bottom  of  the  vessel.  Now  during  this  process  the  quantity 
of  oxygen  will  be  diminished,  but  if  the  scales  mentioned  be 
collected,  they  will  be  found  to  weigh  just  as  much  more  than 
the  original  wire  expended,  as  there  is  of  oxygen  lost  or  com- 
bined with  them.  A  chemist  can  separate  this  iron  and  oxygen, 
and  exhibit  them  apart  as  before,  without  change.  Again,  if 
iron  and  sulphur  in  certain  proportions  be  heated  together,  they 
unite  with  vivid  combustion,  but  the  product  weighs  exactly  as 
much  as  the  original  ingredients. 

While  every  instance  of  combustion  is  thus  only  a  case  of 
chemical  union,  going  on  with  such  intensity  of  action  as  to  pro- 
duce incandescence,  still,  according  to  the  nature  of  the  sub- 
stances combining,  the  appearance  produced  varies  much.  It 
may  be,  for  instance,  with  flame  or  without  flame.  The  great 
combining  substance  in  nature,  that  is  to  say,  the  most  univer- 
sally distributed,  is  oxygen,  of  which  the  name  is  now  become 
familiar  even  to  the  ears  of  the  unlearned.  It  forms  four-fifths 
of  the  substance  of  water  and  one-fifth  of  our  atmosphere,  being 
on  the  latter  account  present  every  where,  and  ready  to  unite 
itself  with  any  matter  exposed  to  it  at  the  necessary  temperature. 
Now  of  substances  burning  in  air,  those  which  are  originally 
aeriform,  as  coal  gas,  or  which  on  being  heated  are  vaporized 
or  rendered  aeriform  before  the  union  takes  place,  as  oil  or  wax, 
assume  the  appearance  of  flame;  viz.  the  aeriform  particles 
usually  invisible  are  raised  to  the  incandescent  temperature;  but 
when  the  substance  combining  with  the  oxygen  remains  solid, 
while  its  particles  are  gradually  lifted  away  by  the  oxygen  act- 

14 


106  HEAT. 

ing  only  at  the  surface  of  their  mass,  it  appears  during  the  whole 
time  only  as  a  red-hot  stone.  The  latter  is  the  case  of  charcoal, 
coke,  Welch  stone  coal,  &c.,  while  in  the  case  of  wood,  com- 
mon coal,  &c.,  a  greater  or  less  portion  of  the  inflammable  mat- 
ter is  by  the  heat  of  the  combustion  converted  into  vapour,  and 
produces  the  beautiful  appearance  of  flame. 

Of  the  substances  called  combustible,  and  thus  called  because 
they  combine  with  oxygen  so  energetically  as  to  become  incan- 
descent, there  are  only  a  few  which  will  begin  to  unite  or  burn 
at  the  common  temperature  of  our  globe,  the  others  requiring 
to  be  at  some  higher  and  peculiar  temperature.  Thus  phospho- 
rus begins  to  burn  at  150°,  sulphur  at  550°,  charcoal  at  750°, 
hydrogen  at  800°,  &c.;  it  appearing  that  up  to  these  temperatures 
the  attraction  of  the  atoms  of  the  substances  among  themselves 
is  sufficient  to  resist  the  other  attraction,  or  that  of  oxygen.  But 
when  the  combustion  once  begins,  the  temperature,  from  the 
effect  of  the  combustion  itself,  rises  instantly  much  beyond  the 
degree  necessary  for  the  commencement  of  the  process.  Oxy- 
gen and  hydrogen,  which  begin  to  burn  or  combine  at  800°, 
produce  a  flame  of  as  intense  heat  as  human  art  can  excite. 

On  the  circumstance  that  bodies  require  to  have  a  certain  pre- 
paratory temperature  before  beginning  thus  to  combine  with  ox- 
ygen, depend  many  important  facts  in  nature  and  art.  Hence  the 
safety  with  which  most  combustibles  may  be  exposed  at  ordinary 
temperatures  to  the  contact  of  atmospheric  air:  otherwise  coal, 
wood,  &c.  in  the  moment  of  being  exposed  to  the  air  would 
catch  fire,  as  really  happens  to  phosphorated  hydrogen  gas;  or 
to  the  metal  called  potassium,  even  when  thrown  into  cold  water, 
the  metal  attracting  the  oxygen  from  the  water  instantly,  and 
with  intense  combustion.  If  a  fire  or  flame  be  so  small  that  it 
does  not  produce  heat  enough  to  maintain  the  inflaming  tempe- 
rature of  the  substance,  the  combustion  will  soon  be  extinguished. 
Thus  a  common  coal  fire,  if  not  watched  by  gathering  together 
the  remnants  to  reduce  the  surface  of  wasteful  radiation,  will  be 
extinguished  long  before  the  fuel  is  all  expended: — but  not  so 
with  a  fire  of  wood  or  of  paper,  which  substances  burn  more 
readily  than  coal.  The  Welch  stone  coal  can  only  be  made  to 
burn  in  very  largo  masses,  or  when  mixed  with  a  more  inflam- 


COMBUSTION.  107 

rnablo  coal  or  other  fuel.  A  substance  placed  in  pure  oxygen 
gas  burns  with  much  greater  intensity,  and  will  begin  burning 
at  a  lower  temperature  than  if  placed  in  atmospheric  air,  which 
contains  only  one-fifth  of  oxygen  and  four-fifths  of  another  sub- 
stance, nitrogen,  which  does  not  aid  the  combustion, — because 
the  nitrogen,  by  absorbing  much  of  the  heat  of  the  combustion, 
lowers  the  temperature.  Iron  wire  will  burn  as  a  taper  in  oxy- 
gen, but  not  in  common  air;  and  a  common  taper  or  flaming 
piece  of  wood  just  extinguished  by  blowing  on  it,  will  imme- 
diately be  rekindled  if  placed  in  oxygen.  Again,  a  lamp  with 
a  very  small  wick,  as  of  one  thread,  and  producing  therefore 
very  little  heat,  will  not  burn  in  cold  weather,  and  at  any  time  will 
be  extinguished  by  a  foreign  body  brought  near  it  so  as  to  cool  it; 
a  small  metallic  nob,  for  instance,  presented  to  it  on  the  end  of  a 
wire,  or  a  metallic  ring  let  down  over  it;  but  if  the  ball  or  ring  be 
hot,  the  effect  will  not  follow.  By  more  powerful  refrigerating 
processes,  even  a  considerable  lamp  or  candle  may  be  put  out. 
These  discoveries  led  Davy  to  the  construction  of  his  miners' 
safety  lamp,  which  is  merely  a  lamp  surrounded  by  a  wire 
gauze,  of  which  the  meshes  are  of  such  size  that  a  flame  of  the 
gas  attempting  to  pass  through  is  so  cooled  by  the  heat-absorb- 
ing and  heat-conducting  power  of  the  metal,  as  to  be  extin- 
guished. A  wire  gauze  gradually  let  down  upon  any  common 
flame,  annihilates  the  part  of  it  which  should  appear  above  the 
gauze;  but  the  combustible  vapour  passing  invisibly  through  the 
gauze  may  be  lighted  afresh  on  its  upper  side.  Oxygen  and 
hydrogen,  which  are  the  constituents  of  water,  when  uniting, 
produce  such  intense  heat  that  the  momentary  expansion  of  the 
newly  formed  water — then  in  the  state  of  steam,  is  such  as  to 
constitute  a  violent  explosion;  and  when  meeting  jets  of  the  two 
gases  at  a  certain  point  allow  a  continued  flame  to  be  formed, 
the  most  refractory  substances  melt  in  it  like  wax  in  a  common 
taper, — yet  these  gases  may  be  kept  mixed  together  in  the  cold 
reservoir  of  a  condensed  air  blow-pipe  without  combining,  and 
when  they  are  set  on  fire  issuing  as  a  jet  from  a  small  opening, 
the  flame  docs  not  travel  inwards  through  the  opening  as 
might  be  feared,  because  it  is  cooled  by  the  metal  of  the  ori- 
fice. 


108  HEAT. 

While  solid  bodies  become  very  visible  or  incandescent  at 
about  1,000°  of  Fahrenheit,  airs,  owing  to  their  tenuity  of  con- 
dition, require  to  be  heated  much  farther  before  they  take  on 
the  vivid  appearance  of  flame;  and  airs  of  light  atoms,  like  hy- 
drogen, require  to  be  heated  still  more  than  heavier  airs.  Thus 
a  wire  held  in  the  pale  blue  flame  of  pure  hydrogen,  becomes 
much  more  luminous  than  the  flame  itself;  and  the  flame  of 
mixed  oxygen  and  hydrogen  escaping  from  a  very  minute  ori- 
fice in  a  glass  tube,  may  itself  be  scarcely  visible,  while  the  ex- 
tremity of  the  tube  heated  by  it  becomes  like  a  brilliant  star. 
Hence  the  light  of  many  flames  may  be  increased  by  placing  a 
wire  gauze  or  other  solid  body  in  the  flame.  Consideration  of 
this  subject  enables  us  to  explain  why  common  coal  gas,  which 
consists  of  hydrogen  holding  a  quantity  of  carbon  in  solution, 
gives  in  burning  a  stronger  light  than  pure  hydrogen,  and  why 
oil  gas,  which  contains  about  twice  as  much  carbon  as  the 
coal  gas,  gives  also  about  twice  as  much  light: — for  it  appears 
that  the  atmospheric  air,  which  first  mixes  with  these  gases 
as  they  issue  to  burn,  is  sufficient  to  combine  with  all  their  hy- 
drogen (which  is  most  strongly  attracts,)  but  not  at  the  same 
time  with  all  their  carbon;  the  particles  of  the  carbon  therefore 
are  separated  or  precipitated  in  the  flame,  and  become  so  many 
solid  particles  most  intensely  heated  and  luminous;  and  after- 
wards when  they  have  ascended  a  little  higher,  they  meet  with 
new  oxygen  and  burn  in  their  turn,  giving  a  second  dose  of  light. 
That  this  decomposition  of  the  gas  really  occurs  is  proved  by 
placing  a  wire  gauze  in  the  flame,  when  we  find  that  if  held 
near  the  middle  of  the  flame,  it  is  immediately  loaded  with  the 
particles  of  charcoal  separated  there,  and  cooled  by  it  so  as  to 
cohere;  while  if  held  at  the  bottom  of  the  flame  where  the  car- 
bon is  not  yet  separated,  it  retains  none,  and  if  held  at  the  top 
of  the  flame,  where  they  are  already  burned,  it  similarly  retains 
none.  A  candle  or  lamp  is  said  to  smoke  when  the  heat  pro- 
duced by  it  is  not  sufficient  to  effect  the  total  combustion  of  the 
carbon  which  rises  in  its  flame. 

When  oxygen  mixed  with  certain  of  the  inflammable  gases 
or  vapours  is  raised  to  a  temperature  even  considerably  below 


FUEL.  109 

that  of  common  burning  or  explosion,  a  union  still  takes  place, 
but  very  slowly,  so  that  the  temperature  never  rises  to  that  ne- 
cessary to  exhibit  flame.  This  phenomenon  has  been  called  in- 
visible combustion.  It  is  remarkably  exemplified  on  plunging 
platinum  or  gold  wire  moderately  heated  into  such  a  mixture: 
the  combination  then  goes  on  in  the  immediate  vicinity  of  the 
hot  wire;  and  although  without  flame,  still  with  sufficient  dis- 
engagement of  heat  to  maintain  the  wire  in  an  incandescent  or 
luminous  state,  as  long  as  there  are  gases  left  to  combine.  Thus 
the  vapour  always  arising  at  a  common  temperature  from  the 
mouth  of  a  phial  of  ether  (ether  consists  chiefly  of  hydrogen 
and  carbon,)  if  made  to  pass  through  a  coil  of  heated  platinum 
wire,  will,  while  by  this  slow  combustion,  combining  with  the 
oxygen  of  the  air  around  it,  give  out  heat  enough  to  keep  the 
wire  so  luminous  as  to  serve  as  a  little  lamp  by  which  to  read 
from  the  dial  plate  of  a  watch  through  the  night.  A  beautiful 
modification  of  this  principle  has  been  adopted  in  the  miners' 
safety  lamp;  and  when  the  air  of  the  mine  is  too  impure  to 
maintain  the  flame,  it  still  suffices  thus  to  produce  a  continued 
light  from  the  incandescent  metal. 

"Fuel." 

Heat  being,  in  the  sense  already  explained,  the  life  of  the 
universe,  and  man  having  command  over  nature  chiefly  by  his 
power  of  controlling  heat,  which  power  again  comes  to  him 
with  the  ability  to  produce  combustion,  it  is  of  great  interest 
to  inquire  what  substances  he  can  most  easily  procure  as  food 
for  combustion,  or  fuel,  as  it  is  called,  and  how  these  may  be 
most  advantageously  employed.  To  speak  on  this  subject  at 
all,  fully  in  reference  to  the  various  arts  of  life,  would  be  to 
compose  an  extensive  work,  but  an  interesting  sketch  may  be 
comprised  within  narrow  limits. 

Although  there  are  a  great  number  of  substances,  which,  in 
the  act  of  their  chemical  union,  occasion  the  heat  and  light  which 
constitute  combustion,  still,  by  far  the  greater  part  of  these, 
in  an  uncombined  state,  are  so  sparingly  distributed  in  nature, 
and  are,  therefore,  procurable  with  such  difficulty,  that  heat 
obtained  by  sacrificing  them  would  be  much  too  expensive  to 


110  HEAT. 

be  within  common  means.  Providence,  however,  has  willed 
that  the  elementary  substance  in  nature  which  has  the  most  en- 
ergetic attraction  for  almost  all  other  substances,  and  which, 
therefore,  produces  in  uniting  with  them  the  most  intense  heat, 
is  also  the  most  universally  distributed  of  all.  This  substance 
is  oxygen.  It  forms  part  of  our  atmosphere,  and  therefore 
penetrates,  and  is  present  wherever  man  can  exist  or  breathe, 
offering  itself  at  once  to  his  service.  .  Then,  for  the. purpose  of 
combining  with  the  oxygen,  there  are  chiefly  two  other  sub- 
stances also  very  widely  scattered,  and  therefore  easily  pro- 
curable and  cheap.  These  are  carbon  and  hydrogen,  the  great 
materials  of  all  vegetable  bodies,  and  therefore  of  our  forest 
trees,  and  of  coal  beds,  which  seem  to  be  the  remains  of  ante- 
diluvian forests.  Carbon  is  found  nearly  alone  in  hard  coal, 
but  "it  is  united  with  a  large  proportion  of  hydrogen  in  caking 
coal,  wood,  wax,  resins,  tallow,  and  oils.  The  gases  used  for 
illumination  are  merely  hydrogen,  holding  certain  quantities 
of  carbon  in  solution;  and  all  bodies  which  burn  with  flame 
give  out  such  gases  in  the  act  of  combustion.  In  the  great 
mass  of  the  earth,  as  known  to  man,  the  stones,  earths,  and 
water,  forming  its  surface,  are  already  combinations  of  oxygen 
with  other  substances,  and  are  therefore  not  in  a  state  to  pro- 
duce fresh  combustion;  but  carbon  and  hydrogen,  by  various 
processes  of  vegetable  and  animal  life,  are  in  numberless  situa- 
tions becoming  accumulated,  so  as  to  be  fit  for  fuel: — as  by 
other  processes  the  atmosphere  is  always  preserved  with  its 
due  proportion  of  oxygen. 

The  name  fuel  is  given  only  to  the  substances  which  com- 
bine with  oxygen,  and  not  to  the  oxygen  itself,  probably  be- 
cause the  former  being  solid  or  liquid,  and  therefore  more  ob- 
vious to  sense,  were  known  as  producers  of  combustion  long 
before  the  existence  of  the  aeriform  ingredient  was  even  sus- 
pected. 

Oils,  fat,  wax,  &c.  being  or  becoming  in  their  combustion 
aeriform,  exhibit  the  appearance  of  flame,  as  already  explained, 
and,  hence,  are  chiefly  used  for  the  purpose  of  giving  light. 
Wood,  again,  and  coal,  are  more  frequently  used  for  mere  heat- 
ing. But  the  chemist's  lamp  for  distilling  and  evaporating,  his 


FUEL.  Ill 

common  blow-pipe  for  directing  the  point  of  a  (lame  upon  any 
substance  to  melt  it,  and  his  condensed-air  blow-pipe,  whose 
flame  of  oxygen  and  hydrogen  is  capable  of  melting  the  most 
refractory  substances,  prove  that  it  is  chiefly  the  expense  of 
the  former  kinds  of  fuel  which  has  nearly  limited  them  to  the 
office  of  light-giving.  Lately  an  important  application  of  oil 
or  fat  as  heat-giving  fuel  has  been  made  in  a  general  cooking 
apparatus,  which  promises  to  effect  a  considerable  diminution 
of  house-keeping  expense. 

Wood  was  the  common  fuel  of  the  early  world  when  coal 
mines  were  not  yet  known,  and  still  in  many  countries  it  is  so 
abundant  as  to  be  the  cheapest  fuel.  Charcoal  is  the  name 
given  to  what  remains  of  wood  after  it  has  been  heated  in  a 
close  place,  during  which  operation  the  hydrogen  and  other  mi- 
nor ingredients  are  driven  away  in  the  form  o'f  vapour.  Char- 
coal is  nearly  pure  carbon.  Coke,  again,  is  the  carbon  obtained 
by  a  similar  preparation  of  coal.  The  wood  and  coal,  if  simi- 
larly heated  in  the  air,  would  burn  or  combine  with  the  Oxy- 
gen of  the  air;  but  heated  in  a  vessel  or  place  which  excludes 
air,  they  merely  give  out  their  more  volatile  parts. 

Good  coal,  where  it  abounds,  is  now  for  ordinary  purposes 
by  much  the  cheapest  kind  of  fuel;  and  since,  within  a  few 
years,  men  have  learned  to  obtain  from  it  separately,  and  to 
use,  instead  of  oil  and  wax,  its  illuminating  gas,  viz.  its  hy- 
drogen, holding  in  solution  a  little  carbon,  it  has  become  doubly 
precious  to  them.  A  person  reflecting  that  heat  is  the  magic 
power  which  vivifies  nature,  and  that  coal  is  what  best  gives 
heat  for  the  endless  purposes  of  human  society,  cannot  without 
admiration  think  of  the  rich  stores  of  coal  which  exist  treasured 
up  in  the  bowels  of  the  earth  for  man's  use.  And  Britain,  in  this 
respect,  is  singularly  favoured.  Her  coal  mines  are  in  effect 
mines  of  labour  or  power  vastly  more  precious  than  the  gold 
and  silver  mines  of  Peru,  for  they  may  be  said  to  produce 
abundantly  every  thing  which  labour  and  ingenuity  can  pro- 
duce, and  they  have  essentially  contributed  to  make  her  mis- 
tress of  the  industry  and  commerce  of  the  earth.  Britain  has 
become,  to  the  civilized  world  around,  nearly  what  an  ordinary 
town  is  to  the  rural  district  in  which  it  stands,  and  of  this  vast 


112  HEAT. 

and  glorious  city  the  mines  in  question  are  the  coal  cellars, 
stored  at  the  present  rate  of  consumption  for  about  1,000  years; 
a  supply  which,  as  coming  improvements  in  the  arts  of  life 
will  naturally  bring  economy  of  fuel,  or  substitution  of  other 
means  to  effect  similar  purposes, — may  be  regarded  as  exhaust- 
less. 

Coal,  we  can  scarcely  doubt,  is  the  remains  of  antediluvian 
forests,  swept  together  during  the  convulsions  of  nature  into 
deep  valleys,  and  there  afterwards  compressed  and  solidified 
by  superincumbent  deposites  of  earthy  matters,  these  deposites 
being  probably  aided  in  their  operation  by  heat.  In  many 
coal  beds  the  trees  of  former  times  yet  retain  their  form,  so 
that  their  species,  can  be  easily  distinguished,  and  there  are  bu- 
ried among  them  other  vegetable  and  animal  remains  of  con- 
temporaneous inhabitants  of  the  earth.  Coal  is  found  of  diffe- 
rent qualities.  In  some  places  it  is  almost  unmixed  carbon, 
and  exceedingly  solid,  as  if  it  had  been  coked  by  subterranean 
heat.  Such  is  the  stone  coal  of  Wales,  which  in  100  parts 
contains  97  of  pure  carbon,  and  only  three  of  hydrogen  and 
earthy  matter.  In  other  places  the  coal  contains  hydrogen  in 
nearly  as  large  proportion  as  wood  does,  and  so  combined  with 
part  of  the  carbon  as  to  form  the  oily  or  pitchy  substances  ex- 
isting in  the  coal,  and  which,  when  burning,  produce  flame, 
and,  when  rising  unburned,  constitute  smoke. 

The  comparative  values,  as  fuel,  of  different  kinds  of  carbo- 
naceous matter,  have  been  found  on  experiment  to  be  as  in  the 
following  tables. 

1  lb.  of  Melts  of  ice. 

Good  coal  -  -  90  Ibs. 

Coke  -  -  84 

Charcoal  of  wood  -  95 

Wood  -  32 

Peat  -  -  -  19 

Lavoisier,  in  making  experiments  on  combustibles  generally, 
to  ascertain  the  quantities  of  oxygen  expanded,  and  of  heat 
given  out  during  the  combustion  of  a  given  quantity  of  each, 
obtained  the  following  results: — 


IT-EL.  11.; 

1  Ib.  of  Melts  of  ice.  Takes  of  oxygen. 

Hydrogen  gas  -                  -  370  Ibs.  -     -     -     -  7£  Ibs. 

Carburetted  hydrogen  -     85 4 

Olive  oil     -  -  120 ;; 

Wax  -                 -  110 ;; 

Tallow  -  105 3 

Charcoal      -  -     95 2-J 

Phosphorus  -  100 1$ 

Sulphur       -  -     25 1 

There  are  some  remarks  with  respect  to  the  using  of  common 
fuel,  which  seem  to  demand  a  place  here. 

•  A  pound  of  coke  produces  nearly  as  much  heat  as  a  pound 
of  coal;  but  we  must  remember  that  a  pound  of  coal  gives  only 
three-quarters  of  a  pound  of  coke,  although  the  latter  is  more 
bulky  than  the  former. 

It  is  wasteful  to  wet  fuel,  because  the  moisture,  on  being  eva- 
porated, carries  off  with  it  as  latent,  and  therefore  useless  heat, 
a  considerable  proportion  of  what  the  combustion  produces. 
It  is  a  very  common  prejudice,  that  the  wetting  of  coal,  by 
making  it  last  longer,  is  effecting  a  great  saving;  but  while,  iu 
truth,  it  restrains  the  combustion,  and  for  a  time  makes  a  bad 
fire,  it  also  wastes  the  heat. 

Coal  containing  much  hydrogen,  as  all  flaming  coal  does,  is 
used  wastefully  when  any  of  the  hydrogen  escapes  without 
burning;  for,  first,  the  great  heat  Which  the  combustion  of  such 
hydrogen  would  produce  is  not  obtained;  and,  secondly,  the 
hydrogen,  while  becoming  gas,  absorbs  still  more  heat  into  the 
latent  state  than  an  equal  weight  of  water  would.  Now,  the 
smoke  of  a  fire  is  the  hydrogen  of  the  coal  rising  in  combina- 
tion with  a  portion  of  carbon.  We  see,  therefore,  that  by  de- 
stroying or  burning  smoke,  we  not  only  prevent  a  nuisance, 
but  effect  a  great  saving.  The  reason  that  common  fires  give 
out  so  much  smoke  is,  either  that  the  smoke,  or  what  we  shall 
call  the  vaporized  pitch,  is  not  sufficiently  heated  to  burn,  or 
that  the  air  mixed  with  it  as  it  ascends  in  the  chimney,  has  al- 
ready, while  passing  through  the  fire,  been  deprived  of  its 
free  oxygen.  If  the  pitch  be  very  much  heated,  its  ingredi- 

15 


114  HEAT. 

ents  assume  a  new  arrangement,  becoming  transparent,  and 
constituting  the  common  coal  gas  of  our  lamps;  but,  at  lower 
temperatures,  the  pitch  is  seen  jetting  as  a  dense  smoke,  from 
cracks  or  openings  in  the  coal — a  smoke,  however,  which  im- 
mediately becomes  a  brilliant  flame  if  lighted  by  a  piece  of 
burning  paper  or  the  approximation  of  the  combustion.  The 
alternate  bursting  out  and  extinction  of  these  burning  jets  of 
pitchy  vapour,  contribute  to  render  a  common  fire  an  object  so 
lively,  and  of  such  agreeable  contemplation  in  the  winter  even- 
ings. When  coal  is  first  thrown  upon  a  fire,  a  great  quantity 
of  vaporized  pitch  escapes  as  a  dense  cold  smoke, — too  cold  to 
burn,  and  for  a  time  the  flame  is  smothered,  or  there  is  none; 
but  as  the  fresh  coal  is  heated  its  vapour  reproduces  the  flame 
as  before.  In  close  fire  places,  those,  viz.  of  great  boilers,  as 
of  steam  engines,  brewing,  and  dyeing  apparatus,  &c.,  all  the 
air  which  enters  after  the  furnace  door  is  shut,  must  pass 
through  the  grate  and  the  burning  fuel  lying  on  it,  and  there 
its  oxygen  is  consumed  by  the  red  hot  coal  before  it  ascends 
to  where  the  smoke  is.  The  smoke,  therefore,  however  hot, 
passes  away  unburnt,  unless  sometimes,  as  over  foundery  fur- 
naces, where  the  heat  is  very  great  indeed,  and  it  burns  as  a 
flame  or  great  lamp  at  the  chimney-top  on  reaching  the  oxy- 
gen of  the  open  atmosphere.  .• 

There  have  been  many  modes  proposed  of  destroying  smoke: 
one  has  been  to  admit,  by  a  suitable  opening,  a  certain  quantity 
of  fresh  air  to  the  space  above  the  fire,  the  oxygen  of  which 
air  may  inflame  the  smoke.  At  a  certain  point  of  time  after 
the  addition  of  fresh  fuel,  this  plan  succeeds,  and  for  the  mo- 
ment effects  a  saving  of  fuel;  but  the  difficulty  of  admitting 
just  the  quantity  of  air  required  to  suit  the  varying  demand  for 
it,  has  not  been  overcome,  and  hence  from  there  being  no 
saving  on  the  whole,  the  plan  has  been  abandoned.  When  just 
enough  air  entered,  the  flame  produced  gave  so  intense  a  heat 
as  in  several  cases  to  have  burned  or  destroyed  the  parts  of  va- 
luable boilers  exposed  to  it;  and  when,  on  the  contrary,  too 
much  air  entered,  it  injuriously  cooled  the  boiler.  The  con- 
trivance at  present  most  commonly  adopted  for  burning  smoke 
is  that  of  Mr.  Brunton,  viz.  a  circular  fire  grate,  kept  turning 


i;i..  115 

like  a  horizontal  wheel,  and  on  which  coal  is  by  machinery 
made  to  fall  in  a  gradual  manner,  so  as  to  be  uniformly  spread 
over  it.  The  coal  falls  so  gradually,  that  although  there  is  ge- 
nerally a  little  smoke  from  it,  there  is  never  much, — the  oxy- 
gen which  finds  entrance,  through,  and  around  the  grate,  being 
always  in  quantity  the  vsame,  and  nearly  sufficient.  A  smoke- 
consuming  fire  would  be  constructed  on  a  perfect  principle,  in 
which  the  fuel  were  made  to  burn  only  at  the  upper  surface  of 
its  mass,  and  so  that  the  pitch  and  gas  disengaged  from  it,  as 
the  heat  spread  downwards,  might  have  to  pass  through  the 
burning  coals  where  fresh  air  was  mixing  with  them:  thus  the 
gas  and  smoke,  being  the  most  inflammable  parts,  would  burn 
first  and  be  all  consumed.  This  was  the  principle  proposed  in 
a  fire-place  suggested  by  the  author  for  the  great  brewery  of 
Mr.  Meux  in  his  neighbourhood,  and  tried  at  the  time  when 
attempts  were  extensively  made  to  abate  the  nuisance  of  smoke 
in  towns.  The  experiment  proved  the  theoretical  perfection 
of  the  method,  and  that  it  would  produce  a  saving  of  15  or  20 
per  cent,  on  the  expenditure  of  coal;  but  before  a  durable 
grate  of  the  kind  was  completed,  the  Welch  stone-coal  was  in- 
troduced, which  has  97  per  cent,  of  pure  carbon,  and  therefore 
no  pitch  to  evaporate,  and  no  smoke, — and  it  was  at  once  adop- 
ted there  and  in  many  other  places.  Coal  in  a  deep  narrow 
trough,  as  a  b  c  d,  if  lighted  at  its  sur- 
face a  b,  burns  with  a  lofty  flame  as  if 
it  were  the  wick  of  a  large  lamp;  for 
all  the  gas  given  out  from  the  coal  be- 
low, as  that  is  gradually  heated,  passes 
through  the  burning  fuel  and  becomes 
a  flame.  Now,  if  we  suppose  nhany 
such  troughs  placed  together,  with  intervals  between  them,  in 
place  of  the  fire  bars  of  a  common  grate  or  furnace,  there  would 
be  a  perfect  unsmoking  fire  place.  Such  was  that  made  on  the 
occasion  mentioned;  and  although  flimsy  and  imperfect,  as  a 
mere  experimental  apparatus,  it  put  beyond  a  doubt  the  possi- 
bility of  accomplishing  its  object.  The  reason  of  the  vast 
saving  of  fuel  by  such  a  grate  is,  that  the  smoke,  instead  of 
stealing  away  latent  heat—being  yet  itself  the  most  combustible 


116  HEAT. 

and  precious  part  of  the  fuel,  gives  all  its  powers  and  worth  to 
the  purpose  of  the  combustion.  The  coal  rested  on  moveable 
bottoms  in  the  troughs,  and  was  moved  up  like  the  wick  of  a 
lamp,  by  its  screw  : — the  bottoms  might  be  lifted  in  many 
ways.  The  author  believes  that  this  construction,  simplified  as 
much  as  possible,  will  still  be  adopted  for  the  Newcastle  or 
flaming  coal, — the  consequences  would  be  so  important.  The 
principle  has  been  already  extensively  introduced  for  common 
parlour  fires  by  Mr.  Cutler  in  his  stove,  which  is  merely  a 
common  grate,  having  instead  of  bottom  bars  a  deep  box  to 
hold  the  coal  for  a  whole  day,  with  a  moveable  bottom,  which 
lifts  the  coal  up  as  wanted.  From  such  a  fire  there  is  alwa}-s 
ascending  a  long  beautiful  flame;  and  much  more  heat  is  given 
out,  than  from  the  same  quantity  of  coal  burned  in  the  common 
way:  the  chimney  never  requires  sweeping,  for  there  is  abso- 
lutely no  smoke,  and  therefore  no  soot. 

It  is  evident  that  if  a  house  or  apartment  with  the  air  in  it, 
were  once  warmed  to  a  certain  degree,  it  would  for  ever  retain 
its  temperature,  but  for  the  escape  of  heat  through  the  walls 
and  windows,  or  with  the  air  from  within,  whether  passing 
away  as  necessary  ventilation  or  as  waste.  A  perfect  system 
of  heating,  therefore,  would  consist  in  diminishing  as  much  as 
possible  these  causes  of  loss,  with  reference  both  to  the  expense 
of  the  means  and  the  salubrity  of  the  dwelling,  and  in  produ* 
cing  and  distributing  the  heat  judiciously.  It  may  be  asserted 
that  a  fourth  part  of  the  fuel  generally  expended  in  English 
houses,  if  more  skilfully  used,  would  better  secure  comfort  and 
health  than  all  that  is  now  expended.  But  it  does  not  accord 
with  the  character  of  this  general  work  to  enter  into  minute 
detail  on  the  subject.  Remarks  were  made  upon  it  in  vol.  i. 
in  the  chapter  on  "  Pneumatics^  under  the  head  of  "  warm- 
ing and  ventilating,"  and  more  minute  information  may  be  ob- 
tained from  Mr.  Tredgold's  work,  expressly  devoted  to  it. 

The  consideration  of  furnaces,  blow-pipes,  &c.  may  appear 
to  some  so  closely  connected  with  our  present  subject  as  to  de- 
mand a  place  here,  but  by  treating  of  them  we  should  be  en- 
croaching on  the  province  of  the  chemist,  &c.  We  may  state 
generally,  that  furnaces  are  merely  arrangements  of  parts  by 


PROM  FRICTION.  117 

\vhich  coal  or  other  fuel  heated  to  the  degree  at  which  it  com- 
bines rapidly  with  the  oxygen  of  the  atmospheric  air,  is  placed 
in  circumstances  favourable  to  the  rapid  renewal  of  the  air, — 
and  a  common  blow  pipe  is  merely  a  jet  of  air  thrown  from  a 
minute  opening  into  any  flame,  so  as  with  great  precision  to 
direct  the  point  of  the  flame  upon  the  body  to  be  heated.  The 
sand  bath  and  water  bath  of  the  chemist  are  merely  means  of 
ensuring  a  more  uniform  or  steady  temperature: — a  vessel  im- 
bedded in  sand,  so  that  heat  can  reach  it  only  through  the  sand, 
cannot  be  very  suddenly  heated  or  cooled,  because  sand  is  a 
slow  conductor;  and  a  vessel  immersed  in  boiling  water,  can 
never  have  greater  heat  than  212°,  or  the  boiling  heat  of  water. 
For  certain  purposes,  hotter  baths,  as  of  high  pressure  steam, 
or  of  vapour  of  oil  of  turpentine,  or  of  boiling  whale-oil,  have 
been  used.  On  such  subjects,  readers  may  consult  works  on 
"  chemistry  applied  to  the  arts." 

"  Condensation  and  Friction  as  causes  of  heat."  (Read  the 
Analysis,  p.  13.) 

A  soft  iron  nail  laid  upon  an  anvil,  and  receiving  in  rapid 
succession  three  or  four  powerful  blows  of  a  hammer,  becomes 
hot  enough  to  light  a  match,  and  if  longer  hammered,  will  be- 
come incandescent  or  red  hot, — partly  from  the  diminished  vo- 
lume or  condensation  of  the  iron,  on  the  principle  already  ex- 
plained, and  partly  from  the  percussion  or  friction,  in  a  way 
not  yet  well  understood,  but  probably  electrical. 

In  the  familiar  case  of  the  mutual  percussion  of  flint  and 
steel,  small  portions  of  one  or  both  are  struck  off  by  the  vio- 
lence of  the  collision,  in  a  state  of  white  heat,  and  the  particles 
of  the  iron  burn  in  passing  through  the  air: — in  a  vacuum  the 
heated  particles  are  equally  produced,  but  are  scarcely  visible 
from  this  combustion  not  occurring.  In  both  cases,  they  suffice 
to  inflame  gunpowder,  or  to  light  tinder.  When  the  materials 
are  good,  the  shower  of  sparks  from  the  sudden  blow  is  most 
copious  and  brilliant. 

The  heat  produced  by  friction  alone,  without  perceivable  con- 
densation of  the  bodies  concerned,  is  exemplified  in  many  facts. 
Two  dry  branches  kept  strongly  rubbing  against  each  other  by 


118  HEAT. 

the  wind,  have  sometimes  set  a  wood  on  fire.  Savages  light 
their  fires  by  analogous  means.  Men  warm  their  cold  hands  in 
winter,  by  rubbing  them  against  each  other,  or  against  their 
coat  sleeves.  Again,  the  axletree  of  a  heavily  laden  wagon  or 
other  carriage,  if  left  without  oil,  often  inflames.  The  line 
attached  to  a  whale  harpoon,  as  it  runs  over  the  side  of  the 
boat  when  the  huge  monster  dives  after  the  harpoon  has  entered 
his  flesh,  requires  water  to  be  constantly  thrown  on  it  to  prevent 
its  setting  fire  to  the  boat.  A  cable  drawn  very  rapidly  through 
the  hawse  hole  by  the  falling  anchor,  produces  great  heat  there 
and  smoke.  When  a  magnificent  ship  is  launched  from  the 
builder's  yard  into  the  deep,  and  glides  along  the  sloping  beams, 
a  dense  smoke  rises  from  the  points  of  rubbing  contact. 

(i  The  Functions  of  Animal  Life  a  source  of  Heat."  (Read 
the  Analysis,  p.  13.) 

It  is  one  of  the  remarkable  facts  in  nature,  that  living  animal 
bodies,  and  to  a  certain  degree  living  vegetables  also,  have  the 
property  of  maintaining  in  themselves  a  peculiar  temperature, 
whether  surrounded  by  bodies  that  are  hotter  or  colder  than 
they.  Captain  Parry's  sailors,  during  the  polar  winter,  where 
they  were  breathing  air  that  could  freeze  mercury,  still  had 
the  natural  warmth  in  them  of  98°  of  Fahrenheit;  and  the  inha- 
bitants of  India,  where  the  same  thermometer  stands  sometimes 
at  115°  in  the  shade,  have  their  blood  at  no  higher  a  tempera- 
ture. 

It  was  at  one  time  the  favourite  explanation  of  this,  that  ani- 
mal heat  was  produced  in  the  lungs,  during  respiration  from  the 
oxygen  then  admitted.  This  oxygen  combines  with  carbon  from 
the  blood,  and  becomes  carbonic  acid  as  in  combustion,  and  it 
was  supposed  to  give  out  a  portion  of  its  latent  heat,  as  in  ac- 
tual combustion;  which  heat  being  then  spread  over  the  body 
by  the  circulating  blood,  maintained  the  temperature.  We  now 
know,  that  if  such  a  process  assist,  which  it  probably  does, — 
for  the  animal  heat  has  generally  a  relation  to  the  quantity  of 
oxygen  expended  in  any  particular  case,  and  when  an  animal 
being  already  much  heated  needs  no  increase,  very  little  oxy- 
gen disappears, — still  much  of  the  effect  is  dependent  on  the 


ANIMAL  FUNCTIONS  A  SOURCE.  119 

influence  of  the  nerves,  either  directly  or  indirectly,  through 
the  vital  functions  governed  by  them.  Mr.  Brodie,  in  his  im- 
portant experiments  upon  the  subject,  found  that  although  in 
animals  apparently  dead  from  injury  done  to  the  nervous  sys- 
tem, he  could  artificially  continue  the  action  of  respiration,  with 
the  usual  formation  of  carbonic  acid,  still  the  temperature  fell 
very  quickly.  The  maintenance  of  low  temperature  in  an  ani- 
mal immersed  in  an  air  hotter  than  itself,  is  partly  attributable 
to  the  copious  perspiration  and  evaporation  which  then  take 
place,  and  which  absorb  into  the  latent  form  the  excess  of  heat 
then  existing.  Perspiration,  both  from  the  skin  and  internal 
surface  of  the  lungs,  occurs  generally  in  proportion  to  the  ex- 
cess of  heat.  Dogs  and  other  animals,  when  much  heated,  as 
they  cannot  throw  off  or  diminish  their  natural  covering,  in- 
crease the  evaporating  surface  by  protruding  a  long  humid 
tongue. 

The  power  in  animals  of  preserving  their  peculiar  tempera- 
ture has  its  limits.  Intense  cold  coming  suddenly  upon  a  man 
who  has  not  sufficient  protection,  first  causes  a  sensation  of  pain, 
and  then  brings  on  an  almost  irresistible  sleepiness,  which  if 
indulged  would  be  fatal.  Sir  Joseph  Banks  having  gone  on 
shore  one  day  near  the  cold  Cape  Horn,  and  being  fatigued, 
was  so  overcome  by  the  feeling  mentioned,  that  he  entreated  his 
companions  to  let  him  sleep  for  a  little  while.  His  prayer 
granted,  might  have  allowed  that  sleep  to  come  upon  him  which 
ends  not — the  sleep  of  death!  as,  under  similar  circumstances, 
it  came  upon  so  many  thousands  of  the  army  which  Buonaparte 
led  into  Russia,  and  lost  there  during  the  disastrous  retreat 
through  the  snows.  Buonaparte's  celebrated  bulletin  allowed, 
that  in  one  night,  when  the  thermometer  of  Reaumur  stood  at 
19°  below  zero,  30,000  horses  died!  Cold  in  inferior  degrees, 
and  longer  continued,  acting  on  persons  imperfectly  protected 
by  clothing,  &c.,  induces  a  variety  of  diseases,  which  destroy 
more  slowly.  A  great  excess  of  heat,  again,  may  at  once  ex- 
cite a  fatal  apoplexy,  and  heat  in  inferior  degrees,  but  long  con- 
tinued, may  cause  those  fevers,  &c.,  which  prevail  in  warm 
climates,  and  which  are  so  destructive  to  strangers  in  these  cli- 
mates. 


120  HEAT. 

Each  species  of  animal  has  a  peculiar  temperature  natural  to 
it,  and  in  the  diversity  are  found  creatures  fitted  to  live  in  all 
parts  of  the  earth,  what  is  wanting  in  internal  bodily  constitu- 
tion being  found  in  the  admirable  protecting  covering  which  na- 
ture has  provided  for  them — covering  which  grows  from  their 
bodies,  with  form  of  fur  or  feather,  in  the  exact  degree  required, 
and  even  so  as  in  the  same  animal  to  vary  with  climate  and  sea- 
son. Such  covering,  however,  has  been  denied  to  man;  but  the 
denial  is  not  one  of  unkindness: — it  is  the  indication  of  his  su- 
perior nature  and  destinies.  Godlike  reason  was  bestowed  on 
man,  by  which  he  subjects  all  nature  to  his  use,  and  he  was 
left  to  clothe  himself. 

The  human  race  is  naturally  the  inhabitant  of  a  warm  climate, 
and  the  paradise  described  as  Adam's  first  abode,  may  be  said 
still  to  exist  over  vast  regions  about  the  equator.  There  the 
sun's  influence  is  strong  and  uniform,  producing  a  rich  and  warm 
garden,  in  which  human  beings,  however  ignorant  of  the  world 
which  they  had  come  to  inhabit,  would  have  their  necessities 
supplied  almost  by  wishing.  The  ripe  fruit  is  there  always 
hanging  from  the  branches;  of  clothing  there  is  required  only 
what  moral  feelings  may  dictate,  or  what  may  be  supposed  to 
add  grace  to  the  form;  and  as  a  shelter  from  the  weather,  a  few 
broad  leaves  spread  on  connected  reeds  will  complete  an  Indian 
hut.  The  human  family,  in  multiplying  and  spreading  in  all 
directions  from  such  a  centre,  would  find,  to  the  east  and  west, 
only  the  lengthened  paradise,  with  slightly  varying  features  of 
beauty;  but  to  the  north  and  south,  the  changes  of  season,  which 
make  the  bee  of  high  latitudes  lay  up  its  winter  store  of  honey, 
and  send  migrating  birds  from  country  to  country  in  search  of 
warmth  and  food,  would  also  rouse  man's  energies  to  protect 
himself.  His  faculties  of  foresight  and  contrivance  would  come 
into  play,  awakening  industry;  and,  as  their  fruits,  he  would 
soon  possess  the  knowledge  and  the  arts  which  secure  a  happy 
existence  in  all  climates,  from  the  equator  almost  to  the  pole. 
It  is  chiefly  because  man  lias  learned  to  produce  at  will,  and  to 
control,  the  wonder-working  principle  of  heat,  that  in  the  rude 
winter,  which  seems  the  death  of  nature,  he,  and  other  tropical 
animals  and  plants  which  he  protects,  do  not  in  reality  perish — 


COMBUSTION.  121 

even  as  a  canary  bird  escaped  from  its  cage,  or  an  infant  ex- 
posed among  the  snow-hills.  By  producing  heat  from  his  fire, 
he  obtains  a  novel  and  most  pleasurable  sort  of  existence;  and 
in  the  night  while  the  dark  and  freezing  winds  are  howling  over 
his  roof,  he  basks  in  the  presence  of  his  mimic  sun,  surrounded 
by  his  friends  and  all  the  delights  of  society,  while  in  his  store- 
rooms, or  in  those  of  merchants  at  his  command,  he  has  the 
treasured  delicacies  of  every  season  and  clime.  He  soon  be- 
comes aware,  too,  that  the  dreary  winter,  instead  of  being  a 
curse,  is  really  in  many  respects  a  blessing,  by  arousing  from  the 
apathy  to  which  the  eternal  serenity  of  a  tropical  sky  so  much 
disposes.  In  climates  where  labour  and  ingenuity  must  pre- 
cede enjoyment,  every  faculty  of  mind  and  body  is  invigorated; 
and  hence  the  sterner  climates  form  the  perfect  man.  It  is  in 
them  that  the  arts  and  sciences  have  reached  their  present  ad- 
vancement, and  that  the  brightest  examples  have  appeared  of 
intellectual  and  moral  excellence. 


(     133     ) 


PART  FOURTH. 

(Continued.] 
SECTION  II.— ON  LIGHT,  OR  OPTICS. 


ANALYSIS  OF  THE  SECTION. 


Light  is  'an  emanation  from  the  sun  and  other  luminous  bodies  be- 
coming  less  intense  as  it  spreads,  and  which,  by  falling  on  other 
bodies,  andbeing  reflected  from  them  to  the  eye,  renders  them  visible. 
It  moves  with  great  velocity,  and  in  straight  lines  where  there  is 
no  obstacle — leaving  shadows  where  it  cannot  fall.  It  passes  rea- 
dily through  some  bodies — which  are  therefore  called  transparent; 
but  when  it  enters  or  leaves  their  surfaces  obliquely,  it  suffers  at 
them  a  degree  of  bending  or  refrattion  proportioned  to  the  obliqui- 
ty. And  a  beam  of  white  light  thus  refracted  or  bent,  under  cer- 
tain circumstances,  is  resolved  into  beams  of  all  the  elementary  co- 
lours, ivhich,  however,  on  being  again  blended,  become  the  white 
light  as  before. 

Transparent  bodies,  as  glass,  may  be  made  of  such  form  as  to  cause 
all  the  rays  which  pass  through  them  from  any  given  point  to  bend 
and  meet  again  in  another  point  beyond  them; — the  body,  then, 
because  usually  in  form  somewhat  resembling  aflat  bean  or  lentil, 
being  called  a  LENS.  And  when  the  light  thus  proceeding  from 
every  point  of  an  object  placed  before  a  lens  is  collected  in  corres- 
ponding points  behind  it,  a  perfect  image  of  the  object  is  there 
produced,  to  be  seen  on  a  white  screen  placed  to  receive  it,  or  in  the 
air,  by  looking  towards  it  in  a  certain  direction.  Now,  the  most 
important  optical  instruments,  and  even  the  living  eye,  are  merely 
arrangements  of  parts  for  producing  and  viewing  such  an  image 
under  variety  of  circumstances.  Wlicn  this  image  is  received  upon  a 
suitable  white  surface  or  screen  in  a  dark  room,  the  arrangement 


LIGHT. 

2.5  called,  according  to  -minor  circumstances,  a  CAMERA  OBSCURA, 

a  MAGIC  LANTEUX,  OT  SOLAR  MICROSCOPE.        Jind  tilt  EYE  itself  I S 

in  fact,  but  a  small  camera  obscura, — of  which  the  pupil  is  the 
round  opening  or  window  before  the  lens, — enabling  the  mind  to 
judge  of  external  objects  by  the  size,  brightness,  colour,  fyc.,  of 
the  very  minute  but  most  perfect  images  or  pictures  formed  at  the 
back  of  the  eye,  on  the  smooth  screen  of  nerve  called  the  retina. 
The  art  of  painting  aims  at  producing  on  a  larger  scale  such 
a  picture,  and  tvhich  tvhen  afterwards  held  before  the  eye,  and 
reproducing  itself  in  miniature  upon  the  retina,  may  excite  the 
same  impression  as  the  original  objects. — When  the  image  beyond 
a  lens,  formed  as  above  described,  is  viewed  in  the  air  by  looking 
at  it  in  a  particular  direction,  then  there  is  exhibited  the  arrange- 
ment of  parts  constituting  the  TELESCOPE,  or  COMMON  MICROS- 
COPE. 

Light  falling  on  very  smooth  or  polished  surfaces,  is  reflected  so 
nearly  in  the  order  in  which  it  falls,  as  to  appear  to  the  eye  as  if 
coming  directly  from  the  objects  originally  emitting  it, — and  sue] L 
surfaces  are  called  mirrors.  Mirrors  may  be  plane,  convex,  or  con- 
cave; and  certain  forms  will  produce  images  by  reflection,  just  as 
lenses  produce  them  by  refraction;  so  that  there  are  reflecting  tele- 
scopes, microscopes,  $*c.,  as  there  are  refracting  instruments  of 
the  same  kind.  Light,  again,  falling  on  bodies  of  rougher  or  irre- 
gular surface,  or  which  have  other  peculiarities,  is  so  modified  as 
to  produce  all  those  phenomena  of  colour  and  varied  brightness  seen 
among  natural  bodies,  and  giving  them  their  distinctive  characters 
and  beauty. 


"  Light."     (See  the  Analysis.) 

The  phenomena  of  light  and  vision  have  always  been  held 
to  constitute  a  most  interesting  branch  of  natural  science;  whe- 
ther in  regard  to  the  beauty  of  light,  or  its  utility.  The  beauty 
is  seen  spread  over  a  varied  landscape — among  the  beds  of  the 
flower-gardens,  on  the  spangled  meads,  in  the  plumage  of  birds, 
in  the  clouds  around  the  rising  and  setting  sun,  in  the  circles 
of  the  rainbow.  And  the  utility  may  be  judged  of  by  the  re- 
flection, that  had  man  been  compelled  to  supply  his  wants  by 
groping  in  utter  and  unchangeable  darkness,  even  if  originally 
created  with  all  the  knowledge  now  existing  in  the  world,  he 


124  LIGHT. 

could  scarcely  have  secured  his  existence  for  one  day.  Indeed, 
the  earth  without  light  would  have  been  an  unfit  abode  even 
for  grubs,  generated  and  living  always  amidst  their  food.  Eter- 
nal night  would  have  been  universal  death.  Light,  then,  while 
the  beauteous  garb  of  nature,  clothing  the  garden  and  the  mea- 
dow,— glowing  in  the  ruby — sparkling  in  the  diamond, — is 
also  the  absolutely  necessary  medium  of  communication  between 
living  creatures  and  the  universe  around  them.  The  rising  sun 
is  what  converts  the  wilderness  of  darkness  which  night  covered, 
and  which  to  the  young  mind,  not  ye.t  aware  of  the  regularity 
of  nature's  changes,  is  so  full  of  horror,  into  a  visible  and  lovely 
paradise.  No  wonder,  then,  if,  in  early  ages  of  the  world, 
man  has  often  been  seen  bending  the  knee  before  the  glorious 
luminary,  and  worshipping  it  as  the  God  of  Nature.  When 
a  mariner  who  has  been  toiling  in  midnight  gloom  and  tempest, 
at  last  perceives  the  dawn  of  day,  or  even  the  rising  of  the 
rnoon,  the  waves  seem  to  him  less  lofty,  the  wind  is  only  half 
as  fierce,  sweet  hope  beams  on  him  with  the  light  of  heaven, 
and  brings  gladness  to  his  heart.  A  man,  wherever  placed  in 
light,  receives  by  the  eye  from  every  object  around,  from  hill  and 
tree,  and  even  a  single  leaf, — nay,  from  every  point  in  every 
object,  and  at  every  moment  of  time,  a  messenger  of  light  to  tell 
him  what  is  there,  and  in  what  condition.  Were  he  omnipre- 
sent, or  had  he  the  power  of  flitting  from  place  to  place  with 
the  speed  of  the  wind,  he  could  scarcely  be  more  promptly  in- 
formed. And  even  in  many  cases  where  distance  intervenes 
not,  light  can  impart  at  once,  knowledge  which,  by  any  other 
conceivable  means,  could  come  only  tediously,  or  not  at  all. 
For  example,  when  the  illuminated  countenance  is  revealing 
the  secret  workings  of  the  heart,  the  tongue  would  in  vain  try 
to  speak,  even  in  long  phrases,  what  one  smile  of  friendship  or 
affection  can  in  an  instant  convey; — and  had  there  been  no  light, 
man  never  could  have  been  aware  of  the  miniature  worlds  of 
life  and  activity  which,  even  in  a  drop  of  water,  the  microscope 
discovers  to  him;  nor  could  he  have  formed  any  idea  of  the 
admirable  structure  belonging  to  many  minute  objects.  It  is 
light,  again,  which  gives  the  telegraph,  by  which  men  converse 
from  hill  to  hill,  or  across  an  extent  of  raging  sea, — and  which 


EMANATION  PROM  THE  SUN,  &C.  125 

pouring  upon  the  eye  through  the  optic  tube,  brings  intelli- 
gence of  events  passing  in  the  remotest  regions  of  space. 

"Emanation  from  the  sun,"  $c.     (See  the  Analysis,  page 

122.) 

The  relation  of  the  sun  to  light  is  most  strikingly  marked 
in  the  contrast  between  night  and  day;  as  the  relation  between 
combustion  and  light  is  seen  in  the  brilliancy  of  an  illuminated 
hall  or  theatre,  as  compared  with  the  perfect  darkness  when 
the  chandeliers  are  extinguished.  In  tropical  countries,  where 
the  sun  rises  almost  perpendicularly,  and  allows  not  the  long 
dawn  and  twilight  of  temperate  latitudes,  the  change  from 
perfect  darkness  to  the  overpowering  effulgence  of  day,  is  so 
sudden  as  to  be  most  impressive.  An  eye  turned  to  the  east 
has  scarcely  noted  a  commencing  brightness  there,  when  that 
brightness  has  already  become  a  glow;  and  if  clouds  be  floating 
near  to  meet  the  upward  rays,  they  appear  as  masses  of  golden 
fleece  suspended  in  the  sky:  a  little  after  the  whole  atmosphere 
is  bright,  and  the  stream  of  direct  light  bending  round,  makes 
the  lofty  mountain-tops  shine  like  burnished  pinnacles;  then  as 
the  stream  reaches  to  still  lower  and  lower  levels,  the  inhabi- 
tants of  these  in  succession  see  the  radiant  circle  first  rising 
above  the  horizon  like  a  tip  of  flame,  but  soon  displaying,  as  in 
days  of  Pagan  worship,  all  its  breadth  and  glory, — too  bright 
for  the  eye  to  dwell  upon.  With  evening  the  same  appear- 
ances recur  in  a  reversed  order,  ending,  as  in  the  morning  they 
began,  in  complete  darkness. 

Light  emanates  also  from  the  stars,  but  they  are  so  distant  as 
in  that  respect  to  be  of  little  importance  to  this  earth.  And 
there  are  still  other  transient  sources  in  animal  and  vegetable 
nature,  and  among  solar  phosphori,  but  they  do  not  merit  par- 
ticular attention  here. 

There  have  been  two  opinions  respecting  the  nature  of  light: 
one,  that  it  consists  of  extremely  minute  particles  darting  all 
around  from  the  luminous  body;  the  other,  that  the  phenome- 
non is  altogether  dependent  on  an  undulation  among  the  par- 
ticles of  a  very  subtile  elastic  fluid  diffused  through  space, — as 
sound  is  dependent  on  an  undulation  among  air  particles.  Now, 


126  LIGHT. 

if  light  be  particles  darting  around,  their  minuteness  must  be 
wonderful,  as  a  taper  can  fill  with  them  for  hours  a  space  of 
four  miles  in  diameter;  and  with  the  extreme  velocity  of  light, 
if  its  particles  possessed  at  all  the  property  of  matter  called  in- 
ertia, their  momentum  should  be  very  remarkable, — it  being 
found,  however,  that  even  a  large  sun-beam  collected  by  a 
burning-glass,  and  thrown  upon  the  scale  of  a  most  delicate  ba- 
lance, has  not  the  slightest  effect  upon  the  equilibrium.  Such, 
and  many  other  facts  to  be  treated  of  in  subsequent  parts  of 
this  work,  lead  to  the  opinion  that  there  is  an  undulation  of  an 
elastic  fluid  concerned  in  producing  the  phenomenon  of  light. 

"  Becoming  less  intense  as  it  spreads."     (See  the  Analysis^ 

page  122.) 

Any  emanation  from  a  central  point,  in  spreading  through 
wider  space,  becomes  proportionally  thinner  or  less  intense. 
Thus,  if  a  taper  be  placed  in  the  centre  of  a  box,  each  side  of 
which  is  a  foot  square,  all  the  light  must  fall  on  the  sides  of  the 
box,  and  will  have  a  certain  intensity  there; — if  the  taper  be 
then  placed  in  a  box  with  sides  of  two  feet  square,  there  will 
be  only  the  same  quantity  of  light,  but  that  will  be  spread  over 
four  times  the  surface  (a  square  of  two  feet  is  made  up  of  four 
squares  of  one  foot,)  and  will  therefore  on  any  part  of  that 
surface  be  only  one-fourth  part  as  strong  or  intense  as  in  the 
first  box: — and  so  for  any  other  size  of  box  or  space,  the  inten- 
sity diminishing  as  the  square  of  the  distance  increases. 

Hence  four  times  as  much  light  and  heat  fall  upon  a  foot  of 
this  earth's  surface  as  upon  a  foot  of  the  surface  of  the  planet 
Mars,  which  is  twice  as  distant  from  the  sun: — as  four  times 
as  much  light  and  heat  fall  on  a  man  who  is  at  one  yard  from 
the  fire,  as  on  another  who  is  at  two  yards. 

"  Falling  on  other  bodies  makes  them  visible.7'     (Read  the 
Analysis,  page  122.) 

If  the  window  shutter  of  an  apartment  be  perfectly  closed, 
an  eye  there  turns  upon  an  absolute  blank:  it  perceives  nothing. 
If  a  ray  of  the  sun  be  then  admitted,  and  made  to  fall  upon  any 
object,  that  object  becomes  bright,  and  affects  the  eye  as  if  it 


MAKES  BODIES  VISIBLE.  127 

were  itself  luminous.  It  returns  a  part  of  the  light  which  falls 
upon  it,  and  it  is  visible  in  all  directions,  proving  that  it  scat- 
ters the  received  light  all  around.  This  scattered  light,  again, 
falling  on  other  objects,  and  reflected  from  and  among  them  un- 
til absorbed,  like  echo  repeated  many  times  and  lost  between 
perpendicular  rocks,  makes  all  of  them  also  visible,  although  in 
a  less  degree,  and  the  whole  apartment  is  said  to  be  lighted. 
If  the  sun's  ray  be  made  to  fall  upon  a  thing  which  from  its  na- 
ture reflects  much  of  the  light,  as  a  sheet  of  white  paper,  the 
apartment  will  be  well  lighted: — if,  on  the  contrary,  it  be  re- 
ceived on  black  velvet,  which  returns  hardly  any  light,  the 
apartment  will  remain  dark; — and,  again,  if  received  on  a  po- 
lished mirror,  which  returns  nearly  the  whole  light,  but  in  one 
direction  only,  and  therefore  throws  it  upon  some  other  single 
object,  the  effect  will  be  according  to  the  nature  of  that  object^ 
and  nearly  as  if  the  ray  had  fallen  directly  upon  it. 

Now,  all  bodies  on  earth,  and  very  remarkably  the  mass  of 
atmosphere  surrounding  the  earth,  retain  and  diffuse  among 
themselves  for  a  time  the  light  received  directly  from  the  sun, 
and  by  so  doing,  maintain  that  milder  radiance  so  agreeable  to 
the  sight,  which  renders  objects  visible  when  the  sun's  direct 
ray  does  not  fall  upon  them.  But  for  this  fact,  indeed,  all  bo- 
dies shadowed  from  the  sun,  whether  by  intervening  clouds  or 
by  any  other  more  opaque  masses  on  earth,  would  be  perfect- 
ly black  or  dark;  that  is,  totally  invisible.  And  without  an 
atmosphere,  the  sun  would  appear  a  red  hot  orb  in  a  black  sky. 
On  lofty  summits,  where  half  the  atmosphere  is  below  the  le- 
vel, the  direct  rays  of  the  sun  are  painfully  intense,  and  the 
sky  is  of  darkest  blue. 

A  shadow  is  the  name  given  to  the  comparative  darkness  of 
places  or  objects,  prevented  by  intervening  obstacles  from  re- 
ceiving the  direct  rays  of  some  luminous  body  shining  on  the 
things  around.  The  apparent  darkness  of  a  shadow,  however, 
i  is  not  proportioned  to  its  real  darkness,  but  to  the  intensity  of 
the  surrounding  lights.  A  landscape  may  be  very  bright,  CVCR 
when  the  sun  is  veiled  by  clouds,  and  then  little  or  no  shadow 
is  perceived;  but,  as  soon  as  the  clouds  pass  away,  deep  sha- 
dows are  cast  behind  every  projecting  object.  Yet  the  objects 


128  LIGHT. 

and  places  then  appearing  so  dark,  are,  in  reality,  more  illumi- 
nated than  before  the  shadow  existed;  for  they  are  receiving, 
and  again  scattering  new  light  from  all  the  more  intensely  il- 
luminated objects  around  Ihem.  A  finger  held  between  a  can- 
dle and  the  wall,  casts  a  shadow  of  a  certain  intensity;  if  ano- 
ther candle  be  then  placed  in  the  same  line  from  the  wall,  the 
shadow  will  appear  doubly  dark,  although,  in  fact,  more  light 
will  be  reaching  the  eye  from  it  than  before.  If  the  candles 
be  separated  laterally,  so  as  to  produce  two  shadows  of  the  finger, 
but  which  coincide  or  overlap  in  one  part,  that  part  will  be  of 
double  darkness,  as  compared  with  the  remainders.  The  most 
accurate  mode  of  comparing  lights  is  to  place  them  at  different 
distances  from  a  screen  or  wall,  so  as  to  make  them  at  the  same 
time  throw  equally  dark  shadows;  and  then,  according  to  the 
law  of  decreasing  intensity  explained  above,  to  calculate  the 
intensities  of  the  sources  of  light  by  the  difference  of  their 
distances  from  the  wall.  The  eye  judges  very  easily  of  the 
equal  intensity  of  compared  shadows. 

The  real  darkness  of  a  shadow  depends  on  the  number  and 
nature  of  the  light-reflecting  objects  around  it.  Thus,  sha- 
dows are  less  remarkable  opposite  to  any  white  surface,  as  that 
of  a  recently  painted  wall.  The  reason  why  the  moon  when 
eclipsed,  that  is,  as  will  be  afterwards  explained,  when  passing 
through  the  shadow  cast  by  the  earth  on  the  side  away  from 
the  sun,  is  almost  quite  invisible,  is,  that  there  are  no  similar 
bodies  bearing  laterally  on  the  moon  to  share  their  light  with 
it.  And  the  reason  why  our  nights  on  earth  are  darker  than 
the  shadows  behind  a  house  or  rock  in  the  sun-shine  of  day, 
is  merely  that  there  are  not  other  earths  near  us  to  reflect  light 
into  the  great  night-shadow  of  the  earth,  as  there  are  other 
houses  and  rocks  to  illuminate  the  day-shadow  of  these.  The 
moon  is  the  only  light-reflecting  body  which  the  earth  has  near 
it;  and  we  perceive  how  much  less  dark  the  night-shadow  is 
when  the  moon  is  so  placed  as  to  bear  upon  it.  The  eclipsed 
moon,  again,  is  invisible,  because  facing  the  shadowed  part  of 
the  earth;  but  when  the  moon  is  in  the  situation  called  new 
moon,,  the  bright  crescent,  or  part  directly  illuminated  by  the 
sun,  is  always  seen  to  be  surrounding  the  shaded  part,  as  if 


MOVES  WITH  GREAT  VELOCITY. 

holding  the  old  moon  in  its  arms: — that  is,  the  shaded  side  of 
the  moon  is  then,  in  a  degree,  visible  to  us,  because  facing  the 
enlightened  side  of  the  earth. 

Many  persons  have  doubted  whether  the  light  of  the  moon 
could  be  altogether  reflected  light  of  the  sun;  the  moon  ap- 
pearing to  them  more  luminous  than  any  opaque  body  on  earth 
merely  exposed  to  the  sun's  rays.  Their  error  has  arisen  from 
their  contrasting  the  moon  while  returning  direct  sunshine 
with  the  shadows  of  night  on  the  earth  around  them.  But 
could  they  then  see  on  a  hill  near  them,  a  white  tower  or  other 
object  scattering  light  as  when  receiving  the  rays  of  a  meridian 
sun,  that  object  would  appear  to  them  to  be  on  fire,  and  there- 
fore much  brighter  than  the  moon.  The  moon,  when  above 
the  horizon  in  the  day-time,  is  perfectly  visible  on  earth,  and 
is  then  throwing  towards  the  earth  as  much  light  as  during  the 
night;  but  the  day  moon  does  not  appear  more  luminous  than 
any  small  white  cloud,  and  although  visible  every  day  except 
near  the  change,  many  persons  have  passed  their  lives  without 
ever  observing  it.  The  full  moon  gives  to  the  earth  only  about 
a  one-hundred-thousandth  part  as  much  light  as  the  sun. 

"  Light  moves  with  great  velocity."     (See  the  Analysis, 
page  122.) 

The  extraordinary  precision  with  which  the  astronomical 
skill  of  modern  times  enables  men  to  foretell  the  times  of  re- 
markable appearances  or  changes  among  the  heavenly  bodies, 
has  served  for  the  detection  of  the  fact,  that  light  is  not  an  in- 
stantaneous communication  between  distant  objects  and  the 
eye,  as  was  formerly  believed,  but  a  messenger  which  requires 
time  to  travel: — and  the  rate  of  travelling  has  been  ascertained 
in  the  same  way. 

The  eclipses  of  the  satellites  or  moons  of  the  planet  Jupiter 
had  been  carefully  observed  for  some  time,  and  a  rule  was  ob- 
tained which  foretold  the  instants  in  all  future  time  when  the 
satellites  were  to  glide  into  the  shadow  of  the  planet,  and  dis- 
appear, or  again  to  emerge  into  view.  Now,  it  was  found,  that 
these  appearances  took  place  16£  minutes  sooner  when  Jupiter 
was  near  the  earth,  or  on  the  same  side  of  the  sun  with  the 

17 


130  LIGHT. 

earth,  than  when  it  was  on  the  other  side;  that  is  to  say,  more 
distant  from  the  earth  by  one  diameter  of  the  earth's  orbit,  and 
at  all  intermediate  stations  the  difference  diminished  from  the 
16i  minutes,  in  exact  proportion  to  the  less  distance  from  the 
earth.  This  proves  then  that  light  takes  16^  minutes  to  travel 
across  the  earth's  orbit,  and  8$  minutes  for  half  that  distance, 
or  to  come  down  to  us  from  the  sun. 

The  velocity  of  light,  ascertained  in  this  way,  is  such,  that 
in  one  second  of  time,  viz.  during  a  single  vibration  of  a  com- 
mon clock  pendulum,  it  would  go  from  London  io  Edinburgh 
and  back  200  times,  and  the  distance  between  these  is  400 
miles.  This  velocity  is  so  surprising  that  the  philosophic  Dr. 
Hooke,  when  it  was  first  asserted  that  light  was  thus  progres- 
sive, said  he  could  more  easily  believe  the  passage  to  be  abso- 
lutely instantaneous,  even  for  any  distance,  than  that  there 
should  be  a  progressive  movement  so  inconceivably  swift.  The 
truth,  however,  is  now  put  quite  beyond  a  doubt  by  many  col- 
lateral facts  bearing  upon  it. 

As  regards  all  phenomena  upon  earth,  they  may  be  regarded 
as  happening  at  the  very  instant  when  the  eye  perceives  them; 
the  difference  of  time  being  too  small  to  be  appreciated: — for, 
as  shown  in  the  preceding  paragraph,  if  our  sight  could  reach 
from  London  to  Edinburgh,  we  should  perceive  a  phenome- 
non there  in  the  four-hundredth  part  of  a  second  after  its  oc- 
currence. 

It  is,  hence,  usual,  and  not  sensibly  incorrect,  when  we  are 
measuring  the  velocity  of  sound,  as  when  a  cannon  is  fired,  by 
observing  the  time  between  the  flash  and  the  report,  to  suppose 
that  the  event  takes  place  at  the  very  moment  when  it  is  per- 
ceived by  the  eye. 

In  using  a  telegraph,  no  sensible  time  is  lost  on  account  of 
light  requiring  time  to  travel.  A  message  can  be  sent  from 
London  to  Portsmouth  in  a  minute  and  a  half;  and  at  the  same 
rate  a  communication  might  pass  to  Rome  in  about  half  an 
hour,  to  Constantinople  in  forty  minutes,  to  Calcutta  in  a  few 
hours,  and  so  on.  A  telegraph  is  any  object  which  can  be 
made  to  assume  different  forms  or  appearances  at  the  will  of 
an  attendant,  and  so  that  the  changes  may  be  distinguished  at 
a  distance.  A  pole  with  moveable  arms  is  the  common  con- 


i 


PROCEEDS  IN  STRAIGHT  LINES.  131 

struction,  each  position  standing  for  a  letter,  or  cipher,  or 
word,  or  sentence,  as  may  be  agreed  upon.  Telegraphic  sig- 
nals between  ships  at  sea  are  generally  made  by  a  few  flags,  the 
meanings  of  each  being  varied  by  the  mast  on  which  it  is  hoist- 
ed, and  by  its  combination  with  others. 

"  Light  proceeds  in  straight  lines,"  $c.     (Read  the  Analy- 
sis, page  122.) 

Our  very  notion  of  a  straight  line  is  taken  from  the  direc- 
tion in  which  light  moves: — but  we  can  verify  a  line  so  ob- 
tained by  other  means,  as  by  stretching  a  cord  between  the  two 
extremes,  or  by  suspending  a  weight  by  a  cord,  and  making  a 
moveable  solid  measure  to  correspond  with  this,  which  mea- 
sure may  be  used  in  any  other  case. 

We  can  see  through  a  straight  tube,  but  not  through  a  crook- 
ed one.  The  vista  through  a  long  straight  tunnel  is  striking 
as  an  illustration  of  this  fact,  and  of  the  diminution  in  the  ap- 
parent size  of  objects  as  they  are  more  distant.  If  a  person 
enter  one  end  of  the  canal  tunnel,  two  miles  long,  cut  through 
the  chalk  hills  near  Rochester,  to  join  the  Thames  and  Med- 
way  rivers,  the  opening  at  the  distant  end  is  seen  as  a  minute 
luminous  speck,  having  the  form  of  the  general  arch,  and  ap- 
pearing in  the  centre  of  the  shade  to  an  eye  placed  in  the  cen- 
tre; and  a  person  who  has  advanced  half  way  through  the  tun- 
nel, may  see  the  luminous  speck,  at  each  end,  then  appearing 
a  little  larger  than  in  the  former  case. 

In  taking  aim  with  gun  or  arrow,  we  are  merely  trying  to 
make  the  projectile  go  to  the  desired  objects  nearly  by  the  path 
along  which  the  light  comes  from  the  object  to  the  eye. 

A  carpenter  looks  along  the  edge  of  a  plank,  &c.  to  see  whe- 
ther it  be  straight. 

Because  light  moves  in  straight  lines,  if  a  number  of  similar 
objects  be  placed  in  a  row  from  the  eye,  the  nearest  one  hides 
the  others.  In  a  wood  or  a  city,  a  person  sees  only  the  trees 
or  houses  that  are  near. 

Some  ignorant  people  believe  that  a  squinting  person  can  see 
round  a  corner,  as  they  believe  that  a  crooked  gun  can  shoot 
round  a  corner. 


!;"!:>  LIGHT 

All  astronomical  and  trigonometrical  observations  are  made 
on  the  faith  of  this  property  of  light,  the  observer  holding  that 
any  object  is  situated  from  him  in  the  direction  in  which  the 
light  comes  to  him  from  it.  When  the  mariner,  after  watching 
for  hours  in  cloudy  weather,  has  caught  a  glimpse  of  the  sun  or 
a  star  through  his  sextant  glass,  he  has  ascertained  his  place 
among  the  trackless  waves,  and  boldly  advances  through  the 
midst  of  hidden  dangers.  And  the  beam  from  the  light  house 
looking  from  the  rocky  height  over  the  sea,  would  be  useless  if 
the  light  from  it  came  not  in  a  straight  line. 

"  Leaving  shadows  where  it  cannot  fall."     (See  the  Ana- 
lysis, page  122.) 

The  form  of  shadows  proves  that  light  moves  in  straight 
lines,  for  the  outline  of  the  shadow  is  always  correctly  that  of 
the  object  as  seen  from  the  luminous  body.  If  the  light  bent 
round  the  body,  this  could  not  be. 

The  shadow  of  a  face  on  the  wall  is  a  correct  profile. 

As  a  wheel  presented  edgeways  to  the  eye  appears  only  like 
a  broad  line,  but  becomes  oval  or  round  as  it  is  more  turned,  so 
a  wheel  presented  edgeways  to  the  sun  or  other  light,  casts  a 
linear  shadow  on  the  wall  behind  it,  the  shadow  becoming  oval 
or  round  as  the  position  is  changed. 

A  globe,  a  cylinder,  a  cone,  and  a  flat  circle,  will  all  throw 
the  same  form  of  shadow  if  held  with  their  axes  pointing  to  the 
luminous  body,  and  therefore  by  the  shadow  only,  these  objects 
could  not  be  distinguished. 

The  figure  of  a  rabbit  cut  in  pasteboard,  will  throw  the  same 
sbadow  on  the  wall  as  the  animal  itself ;  and,  again,  that  sha- 
dow may  be  perfectly  imitated  by  a  certain  position  of  the  two 
hands  joined,  as  is  known  to  those  who  find  pleasure  in  wit- 
nessing the  surprise  and  delight  of  infancy  made  to  behold  such 
a  shadow  mimicking  the  actions  of  life. 

A  man  under  the  vertical  sun  stands  upon  his  little  round 
shadow;  but  as  the  sun  declines  in  the  afternoon,  the  shadow 
juts  out  on  the  opposite  side,  and  at  last  may  extend  over  a 
whole  field. 

A  distant  cloud  which  appears  to  the  eye  of  an  observer  only 


FORM   OP  SHADOWS.  133 

as  a  line  in  the  sky,  maybe  shadowing  a  whole  region;  for 
clouds  generally  form  in  level  strata,  and  when  viewed  at  a 
distance  are  seen  edgeways. 

The  velocity  of  the  wind  may  be  ascertained  by  marking  the 
time  which  the  shadow  of  a  cloud  takes  to  pass  over  a  plain  or 
other  space  of  known  dimension. 

All  the  heavenly  bodies  of  the  solar  system  cast  a  shadow 
beyond  them  on  the  side  opposite  to  the  sun,  as  is  seen  when 
any  body  previously  visible  passes  where  that  shadow  is.  The 
satellites  or  moons  of  Jupiter,  when  they  suddenly  disappear  to 
our  glasses,  have  generally  only  plunged  into  the  shadow  of  the 
planet,  and  are  not  hidden  behind  its  body,  as  many  suppose. 
When  our  own  moon  is  the  subject  of  that  phenomenon  so  aw- 
ful in  the  early  ages  of  the  world,  an  eclipse,  she  is  only  pass- 
ing through  the  round  shadow  which  the  earth  casts  beyond  it. 

When  the  luminous  centre  is  larger  than  the  body  which 
casts  the  shadow,  the  shadow  will  he  less  than  the  body.  This 
is  true  of  the  shadows  of  all  the  planets  and  of  the  earth,  be- 
cause they  are  less  than  the  sun. 

On  the  contrary,  if  the  light-giving  surface  is  smaller  than 
the  opaque  body,  the  shadow  will  be  larger  than  the  body. 
The  shadow  of  a  hand  held  between  a  candle  and  the  wall  is 
gigantic;  and  a  small  pasteboard  figure  of  a  man  placed  near  a 
narrow  centre  of  light,  throws  a  shadow  as  big  as  a  real  man. 
The  latter  fact  has  been  amusingly  illustrated  by  the  art  of 
making  phantasmagoric  shadows. 

When  the  surface  which  receives  a  shadow  is  not  directly  ex- 
posed to  the  light,  the  shadow  may  be  much  larger  than  the  ob- 
ject, even  although  the  sun  himself  be  throwing  the  light;  as  is 
seen  when  a  slightly  projecting  roof  shadows  from  the  high 
sun  of  summer  noon  the  whole  front  of  a  house,  or  as  is  proved 
by  the  long  evening  shadows  of  all  countries. 

"Light  passes  readily  through  some  bodies — which  are 
therefore  called  transparent;  but  when  it  enters  or  leaves 
their  surfaces  obliquely,  its  course  is  bent."  (Read  the 
Analysis,  page  122.) 

It  may  well  excite  the  surprise  of  inquirers  that  light,  of 


134  LIGHT. 

which  the  constituent  particles  are  so  inconceivably  minute, 
should  still  be  able  to  dart  readily  and  in  every  direction  through 
great  masses  of  solid  matter,  but  such  is  the  truth.  Thick 
plates  of  solid  glass,  blocks  of  rock  crystal,  mountains  of  ice, 
&c.,  are  instantly  pervaded  by  the  beam  of  the  sun. 

What  it  is  in  the  constitution  of  one  mass  as  compared  with 
another,  which  fits  the  former  to  transmit  light,  and  the  latter 
to  obstruct  it,  we  cannot  clearly  explain,  but  we  perceive  that 
the  arrangement  of  the  particles  has  more  influence  than  their 
peculiar  nature.  Nothing  is  more  opaque  than  thick  masses  of 
the  metals,  but  nothing  is  more  transparent  than  equally  thick 
masses  of  the  same  metals  in  solution,  nor  than  the  glasses  of 
which  a  metal  forms  a  large  proportion.  The  thousand  salts 
formed  by  the  union  of  the  metals  or  earths  with  the  diluted 
acids,  are  all  transparent,  when,  in  cooling  from  the  fluid  to  the 
solid  state,  their  particles  have  been  allowed  to  arrange  them- 
selves according  to  the  laws  of  their  mutual  attraction,  that  is 
to  say,  to  form  crystals;  but  the  same  substances  in  other  states, 
as  when  reduced  to  powder,  are  opaque.  Even  the  pure  me- 
tals themselves,  when  reduced  to  leaves  of  great  thinness,  are 
transparent,  as  may  be  perceived  by  looking  at  a  lamp  through 
fine  gold  leaf.  It  is  to  be  remarked,  however,  that  even  the 
most  transparent  bodies  intercept  a  considerable  part  of  the 
light  which  enters  them:  a  depth  of  seven  feet  of  pure  water 
intercepts  about  one  half,  so  that  the  bottom  of  the  sea  is  very 
dark.  And  of  the  sun's  light,  when  passing  obliquely,  through 
the  atmosphere  towards  the  earth,  only  a  small  part  arrives. 

Light  having  once  entered  a  transparent  mass  of  uniform  na- 
ture, passes  forward  in  it  as  straightly  as  in  a  vacuum;  but  at 
the  surface,  whether  on  entering  or  leaving  it,  if  the  passage  be 
oblique,  and  if  the  mass  be  of  a  different  density  from  the 
transparent  medium  around  it,  a  very  curious  and  most  impor- 
tant phenomenon  occurs,  viz.  the  light  suffers  a  degree  of  bend- 
ing from  its  antecedent- direction,  or  a  refraction,  proportioned 
to  the  obliquity. 

But  for  this  fact,  which  to  many  persons  might  at  first  ap- 
pear a  subject  of  regret,  as  preventing  the  distinct  vision  of  ob- 
jects through  all  transparent  media,  light  could  have  been  of 


REFRACTION.  135 

little  utility  to  man.  There  could  have  been  neither  lenses  as 
now,  nor  any  optical  instruments,  as  telescopes  and  microscopes, 
of  which  lenses  form  a  part;  nor  even  the  eye  itself. 

Light  falling  from  the  air  directly  or  perpendicularly  upon  a 
surface  of  water,  glass,  or  any  such  transparent  body,  passes 
through  without  suffering  the  least  bending;— -a  ray  for  instance 

shot  from  a  to  the  point  c,  in  the  sur- 
face of  a  piece  of  glass  g  h,  would 
reach  directly  across  to  b;  but  if  the 
,  ray  fell  obliquely,  as  from  d  to  c, 
then,  instead  of  continuing  in  its  first 
direction,  and  going  on  to  i  and  k,  it 
would  at  the  moment  of  its  entrance 
be  bent  downwards  into  the  path  c  e, 
nearer  to  a  line  c  o,  called  the  per- 
pendicular to  the  surface  at  the  point 

of  entrance, — and  then  moving  straightly  while  in  the  substance 
of  the  glass,  it  would,  when  it  passed  out  again  at  e,  in  the  op- 
posite surface,  be  bent  just  as  much  as  at  first,  but  in  the  con- 
trary direction,  or  away  from  a  similar  perpendicular  at  that 
surface,  viz.  into  the  line  e  f  instead  of  e  n.  A  ray  therefore 
passing  obliquely  through  a  transparent  body  of  parallel  sur- 
faces, has  its  course  shifted  a  little  to  one  side  of  the  original 
course,  but  still  proceeds  in  the  same  direction,  or  in  a  line  pa- 
rallel to  the  first — as  here  shown  in  the  line  e  f,  parallel  and 
near  to  the  line  i  k. 

The  degree  of  bending  or  refraction  of  light  in  traversing  a 
transparent  surface  is  ascertained  by  comparing  the  obliquity  of 
its  approach  to  the  surface  with  the  obliquity  of  its  departure 
after  passing;  and  for  this  purpose  a  line  is  supposed  to  be 
drawn  perpendicularly  through  the  surface  at  the  point  where 
the  ray  passes  (as  a  b  in  the  above  figure  drawn  through  cy 
where  the  ray  d  c  passes,)  and  the  relative  positions  of  the  ray 
to  this  line  on  both  sides  of  the  surface,  are  easily  ascertained. 
Thus  the  line  a  d,  drawn  from  any  point  of  such  perpendicular 
to  the  ray  before  passing,  is  a  measure  of  the  original  obliquity 
or  angular  distance  of  the  ray,  and  is  called  the  sine  of  the 
angle  of  incidence,  and  the  other  line  o  e  drawn  from  a  cor- 


136  LIGHT. 

responding  point  of  the  perpendicular  to  the  ray  after  the  pass- 
•ing,  is  a  measure  of  the  obliquity  after  refraction,  and  is  called 
the  sine  of  the  angle  of  refraction: — by  comparing  these 
two  lines  in  any  case,  the  problem  is  solved. 

When  light  passes  obliquely  from  air  into  water,  the  refrac- 
tion or  bending  produced  is  such,  that  the  line  a  d  measuring 
the  obliquity  before  refraction,  is  always  longer  than  the  line 
o  e  measuring  it  after  refraction,  by  nearly  one-third  of  the  latter, 
and  the  refractive  power  of  water  is  therefore  signified  by  the 
index.  1  §  or  1.33;  as  in  like  manner  the  greater  refractive  power 
of  common  glass  has  the  index  !£,  that  of  diamond  the  index 
2|,  and  so  on.  And  it  is  important  to  remark,  that  for  the 
same  substance,  whatever  relation  holds  between  the  obliquity 
of  a  ray  and  the  refraction  in  any  one  case,  the  same  holds  for 
all  cases.  If,  for  instance,  where  the  obliquity,  as  measured  by 
its  sine,  is  40,  and  the  refraction  is  half,  or  20,  then  in  the 
same  substance  an  obliquity  of  10  will  occasion  a  refraction  of 
5,  and  an  obliquity  of  4  will  occasion  a  refraction  of  2;  and 
so  on. 

As  a  general  rule,  the  refractive  power  of  transparent  sub- 
stances or  media  is  proportioned  to  their  densities.  It  increases, 
for  instance,  through  the  list  of  air,  water,  salt,  glass,  &c. 
But  Newton,  while  engaged  in  his  experiments  upon  the  sub- 
ject, observed  that  inflammable  bodies  had  greater  refractive 
power  than  others,  and  he  then  hazarded  the  conjecture,  almost 
of  inspired  sagacity,  and  which  chemistry  has  since  so  remark- 
ably verified,  that  diamond  and  water  contained  inflammable 
ingredients.  We  now  know  that  diamond  is  merely  crystal- 
lized carbon,  and  that  water  consists  altogether  of  hydrogen  or 
inflammable  air  and  oxygen.  Diamond  has  nearly  the  greatest 
light-bending  power  of  any  known  substances,  and  hence  comes 
in  part  its  brilliancy  as  a  jewel. 

No  good  explanation, has  been  given  of  the  singular  fact  of 
refraction;  but  to  facilitate  the  conception  and  remembrance  of 
it,  we  say  that  it  happens  as  if  it  were  owing  to  an  attraction  be- 
tween the  light  and  the  refracting  body  or  medium.  The  light 
approaching  from  d  to  c,  for  instance,  may  be  supposed  to  be 
attracted  by  the  solid  body  below  it,  so  as  at  the  surface  to  be 


(7 


REFRACTION.  137 

bent  into  the  direction  c  e;  and,  again,  on  leaving  the  body  to 
be  still  equally  attracted  and  bent  back,  so  as  to  take  the  direc- 
tion ef,  instead  of  e  n. 

The  following  are  familiar  examples  of  this  bending  of  light  in 
passing  from  one  medium  to  another. 

If  an  empty  basin  or  other  vessel,  b  cf,  be  placed  in  the  sun's 
light,  so  that  the  rays  falling  within  it  may  reach  low  on  the 
side,  as  to  d,  but  not  to  the  bottom, 
$.•  then,  on  filling  the  vessel  with  water, 
rx^&  the  sun  will  be  found  to  be  shining  on 
the  bottom  or  down  to  e,  as  well  as  on 
the  side.  The  reason  of  this  pheno- 
menon is,  that  water  being  a  denser 
medium  than  air,  the  light,  on  enter- 
ing it  at  c,  is  bent  towards  the  perpendicular  at  the  point  of  in- 
cidence, or  cf,  and  so  reaches  the  bottom.  Again,  if  a  coin 
or  medal  were  laid  on  the  bottom  of  such  a  vessel  at  e,  it  would 
not,  while  the  vessel  were  empty,  be  seen  by  an  eye  at  a,  but 
would  be  visible  there  immediately  on  the  vessel  being  filled 
with  water; — because  then,  the  light  leaving  it  in  the  direction 
e  c,  towards  the  edge  of  the  vessel,  would  at  c,  on  passing  from 
the  water  into  air,  be  bent  away  from  the  perpendicular  c  h, 
and  instead  of  going  to  g  would  reach  the  eye  at  a.  The  coin 
moreover  would  appear  to  the  eye  to  be  in  the  direction  c  dy 
instead  of  in  the  true  direction  c  e:  for  the  eye  not  being  able  to 
discover  that  the  light  had  been  Bent  in  its  course,  would  ju*dge 
the  object  to  be  in  the  line  by  which  the  light  came  from  it. 

It  is  thus  because  objects  at  the  bottom  of  water,  when  viewed 
obliquely,  do  not  appear  so  low  as  they  really  are,  that  a  per- 
son examining  a  river  or  pond,  or  any  clear  water,  from  its 
bank,  naturally  judges  its  depth  to  be  less  than  the  truth.  Many 
a  young  life  has  been  sacrificed  to  this  error.  A  person  look- 
ing from  a  boat  directly  down  upon  objects  at  the  bottom  of 
water,  sees  them  in  their  true  places  and  at  their  true  distances, 
but  if  he  view  them  more  and  more  obliquely,  the  appearance 
is  more  and  more  deceiving,  until  at  last  it  represents  them  as 
at  less  than  half  of  their  true  depth. 

18 


1 38  LIGHT. 

The  ship  in  which  the  author  sailed  once  in  the  China  sea, 
before  danger  was  apprehended,  had  entered  by  a  narrow  pas- 
sage into  a  horse-shoe  enclosure  of  coral  rocks.  When  the 
alarm  was  given,  the  predicament  had  become  truly  terrific. 
On  every  side,  in  water  most  singularly  transparent,  as  the 
wave  swelled,  the  rocks  appeared  to  be  almost  at  the  surface  of 
the  water,  and  the  anchor,  which  in  the  first  moments  had  been 
let  go  to  limit  the  danger,  appeared  to  be  lifted  with  them-  It 
was  judged  that  if  the  ship,  then  drawing  24  feet,  or  the  depth 
of  a  two-storied  house,  had  moved  but  a  little  way  in  almost 
any  direction,  she  must  have  met  her  certain  destruction.  On 
sending  boats  around  to  sound  and  to  search,  the  place  of  en- 
trance was  again  discovered,  and  was  safely  traversed  a  second 
time  as  an  outlet  from  that  terrible  prison. 

On  account  of  this  bending  of  light  from' objects  under  water, 
there  is  more  difficulty  in  hitting  them  with  a  bullet  or  spear. 
The  aim  by  a  person  not  directly  over  a  fish  must  be  made  at 
;a  point  apparently  below  it,  otherwise  the  weapon  will  miss  it 
by  flying  too  high.  The  spear  is  sometimes  used  in  this  coun- 
try for  killing  salmon,  but  is  a  common  weapon  among  the  isl- 
?inders  of  the  Atlantic  and  Pacific  Oceans  for  killing  the  alba- 
core;  the  use  of  it,  like  that  of  the  fly  hook  in  England,  afford- 
ing, to  the  fishermen,  good  sport  as  well  as  profit.  The  author 
once  with  much  interest  witnessed  at  St.  Helena  this  employ- 
ment of  the  spear.  A  small  fish  previously  half-killed,  that  it 
might  not  try  to  escape,  was  every  minute  or  two  thrown  upon 
the.water  as  a  bait,  in  the  sight  of  perhaps  a  hundred  great  al- 
bacores,  greedily  waiting  for  it  at  one  side  below,  and  know- 
ing the  danger  to  which  they  exposed  themselves  by  darting 
across  to  seize  it.  Some  albacore  bold  enough,  soon  made  at 
the  mouthful,  apparently  with  the  speed  of  lightning,  but  yet 
with  speed  which  did  not  save  him,  for  every  now  and  then 
the  thrown  spear  met  the  adventurer,  and  held  him  writhing 
there  in  a  cloud  of  his  death-blood.  After  a  victim  so  de- 
stroyed, the  scene  of  action  was  changed. 

The  bending  of  light  when  passing  obliquely  from  water,  is 
also  the  reason  of  the  following  facts.  A  straight  rod  or  stick, 
of  which  a  portion  is  immersed  in  water,  appears  crooked  or 


REFRACTION.  139 

broken  at  the  surface  of  the  water,  the  portion  immersed  seem- 
ing to  be  bent  upwards.  That  part  of  a  ship  or  boat  visible 
under  water,  appears  much  flatter  or  shallower  than  it  really 
is.  A  deep-bodied  fish  seen  near  the  surface  of  water,  appears 
almost  a  flat  fish.  A  round  body  there  appears  oval.  A  gold 
fish  in  a  vase  may  appear  as  two  fishes,  being  seen  as  well  by 
light  bent  through  the  upper  surface  of  the  water,  as  by  straight 
rays  passing  through  the  side  of  the  glass.  To  see  bodies  un- 
der water,  in  their  true  places  and  of  their  true  proportions, 
the  eye  must  view  them  through  a  tube,  of  which  the  distant 
end,  closed  with  plate  glass,  is  held  in  the  water. 

As  light  is  bent  on  entering  from  air  into  water,  glass,  or 
other  substance  denser  than  air,  so  is  it  also  bent  on  coming 
from  void  space  into  the  ocean  of  our  atmosphere.  Hence  none 
of  the  heavenly  bodies,  except  when  directly  over  our  heads, 

are  seen  by  us  in  their  true 
situations.  They  all  appear 
a  little  higher  than  they  real- 
ly are,  as-  when  to  a  specta- 
tor at  d,  supposed  on  the  sur- 
face of  the  earth,  a  star  real- 
ly at  A  appears  to  be  at  ai 
because  its  ray  on  reaching 
the  atmosphere  at  c  is  bent  downwards.  In  astronomical  books 
there  is  always  introduced  a  table  of  refraction,  as  it  is  called, 
showing  what  correction  mast  be  made  on  this  account  for  dif- 
ferent apparent  altitudes.  This  effect  of  our  atmosphere  so- 
bends  the  rays  of  the  sun  that  we  see  him  in  the  morning  be- 
fore he  is  really  above  the  horizon,  and  we  see  him  in  the 
evening  after  he  is  really  below  it,  viz.  the  ray  coming  hori- 
zontally from  'e  to  d,  appears  to  come  from  b,  although  in  truth 
it  comes  from  the  lower  situation  B,  and  is  bent  into  the  level 
line  only  at  e'.  Oar  atmosphere  thus,  by  the  bending  of  light 
as  well  as  by  itself  becoming  luminous,  lengthens  at  dawn  and 
twilight  the  duration  of  the  lovely  day.  As  the  atmosphere  is 
denser  near  the  surface  of  the  earth  than  higher  up,  the  light 
is  more  and  more  bent  as  it  descends,  and  hence  describes  a 


140  LIGHT. 

course  which  is  a  little  curved,  and  therefore  unlike  the  course 
of  light  in  water. 

Certain  states  of  the  atmosphere  depending  upon  its  humi- 
dity, warmth,  &c.,  change  very  considerably  its  ordinary  re- 
fractive power,  hence  in  one  state  of  it,  a  certain  hill  or  island 
may  appear  low  and  scarcely  rising  above  the  intervening 
heights  or  ocean,  while  in  another  state,  the  same  object  shall 
be  seen  towering  above:  and  from  a  certain  station,  a  city  in  a 
neighbouring  valley  may  be  either  entirely  visible,  or  it  may 
show  only  the  tops  of  its  steeples,  as  if  the  bed  on  which  it 
rested  had  sunk  deeper  into  the  earth.  In  days  of  ignorance 
and  superstition,  such  appearances  have  sometimes  excited  a 
strange  interest. 

A  beautiful  phenomenon  is  observable  in  a  day  of  warm  sun- 
shine, owing  to  the  bending  of  light  in  passing  through  media 
of  different  densities.  Black  or  dark-coloured  substances,  by 
absorbing  much  light  and  heat  from  the  sun's  rays,  and  warming* 
the  air  in  contact  with  them,  until  it  dilates  and  rises  in  the 
surrounding  air,  as  oil  rises  in  water,  cause  the  light,  from 
more  distant  objects,  reaching  the  eye  through  the  rarefied  me- 
dium, to  be  bent  a  little;  and  owing  to  the  heated  air  rising  ir- 
regularly under  the  influence  of  the  wind  and  other  causes, 
these  objects  acquire  the  appearance  of  having  a  tremulous  or  a 
dancing  motion.  In  a  warm  clear  day,  the  whole  landscape  at 
last  appears  to  be  thus  dancing. 

The  same  phenomenon  is  to  be  observed  at  any  time,  by 
looking  at  an  object  beyond  the  top  of  a  chimney  from  which 
hot  air  is  rising.  An  illicit  distillery  was  once  discovered  by 
the  exciseman  happening  thus  to  look  across  a  hole  used  as  the 
chimney,  although  charcoal  was  the  fuel,  and  there  was  no 
vestige  of  smoke. 

This  bending  of  light  by  the  varying  states  of  the  atmosphere 
makes  precaution  necessary  in  making  very  nice  geometrical 
observations: — as  in  measuring  base  lines  for  the  construction 
of  maps  or  charts. 

As  it  is  the  obliquity  between  the  passing  ray  and  the  sur- 
face, which  in  any  case  of  refraction  determines  the  degree  of 


REFRACTION. 


141 


bending,  a  body  seen  through  a  medium  of  irregular  surface 
appears  distorted  according  to  the  nature  of  that  surface.  It  is 
because  the  two  surfaces  of  common  window-glass  are  not  per- 
fect planes,  and  not  perfectly  parallel  to  each  other,  as  in  the 
case  of  plate  glass,  that  objects  seen  through  the  former  appear 
generally  more  or  less  out  of  shape;  and  hence  comes  the  ele- 
gance and  beauty  of  plate  glass  windows:  and  hence  the  singu- 
lar distortion  of  things  viewed  through  that  swelling  or  lump 
of  glass  which  remains  where  the  glass-blower's  instrument 
was  attached,  and  which  appears  at  the  centre  of  certain  very 
coarse  panes. 

The  refraction  or  bending  of  light  is  interestingly  exempli- 
fied in  the  effect  of  the  glass  called  a  prism,  viz.  a  wedge  or 
three-sided  rod  of  glass,  such  as  that  of  which  the  end  is  here 

represented  at  b  c.  A 
ray  from  «,  falling  on 
the  surface  at  b,  is  bent 
towards  the  internal  per- 
pendicular, and  therefore 
reaches  c,  but,  on  escaping  again  at  c,  it  is  bent  away  from  the 
external  perpendicular,  and  thus,  with  its  original  deviation 
doubled,  goes  on  to  d. 

The  law  of  light's  bending,  according  to  the  obliquity  with 
which  it  traverses  the  surfaces  of  a  transparent  body,  is  well 
elucidated  by  the  effect  of  what  is  called  a  multiplying  glass; 
that  is  to  say,  a  piece  of  glass  like  .a  bee, 
having  many  distinct  faces  cut  upon  it  at  an- 
gles with  each  other.  If  a  small  object,  a 
coloured  bead  for  instance,  be  placed  at  dr 
an  eye  at  e  will  see  as  many  beads  as  there 
are  distinct  surfaces  or  faces  on  the  glass;  for, 
first,  the  ray,  d  a,  passing  perpendicularly, 
and  therefore  straight  through,  will  form  an 
image  as  if  no  glass  intervened,  then,  the 
rays  from  d  to  the  surface  b  will  be  bent  by 
the  oblique  surface,  and  will  show  the  object 
as  if  it  were  in  the  direction  e  b;  then  the 
light  falling  on  the  still  more  oblique  sur- 
face, c,  will  be  still  more  bent,  and  will  reach 


142  LIGHT. 

the  eye  in  the  direction  c  e,  exhibiting  a  similar  object  also  in 
that  direction — and  so  of  all  the  other  surfaces.  If  the  rela- 
tive places  of  the  eye  and  object  be  changed,  the  result  will  still 
be  the  same.  A  plate  of  glass,  roughened  or  cut  into  cross  fur- 
rows, becomes  a  very  good  screen  or  window  blind,  by  so  dis- 
turbing the  passage  of  light  through  it,  that  objects  beyond  it 
are  not  distinguishable. 

"  <ftnd  a  beam  of  white,  light  thus  made  to  bend,  is  resolved 
into  beams  of  the  various  primary  colours;  which  beams, 
however,  on  being  again  blended,  become  white  light  as 
before.7'  (Read  the  Analysis,  page  122.) 

The  most  extraordinary  fact  connected  with  the  bending  of 
light  is,  that  a  pure  ray  of  white  light  from  the  sun  admitted 
into  a  darkened  room  by  a  hole  in  the  window  shutter,  and 
made  to  bend  by  passing  through  transparent  surfaces  which  it 

meets  very  obliquely  (as 
the  ray  a,  admitted  and 
made  to  bend  by  passing 
through  the  prism  of 
glass,  b  c,  to  fall  upon 
the  wall  at  d,)  instead  of  bending  all  together  and  appearing 
still  as  the  same  white  ray,  is  divided  into  several  rays,  which 
falling  on  the  white  wall,  are  seen  to  be  of  different  most  vi- 
vid colours.  The  original  white  ray  is  said  thus  to  be  ana- 
lyzed or  divided  into  elements. 

This  solar  spectrum,  as  it  is  called,  formed  upon  the  wall, 
consists,  when  the  light  is  admitted  by  a  narrow  horizontal 
slit,  of  four  coloured  patches  corresponding  to  the  slit,  and  ap- 
pearing in  the  order,  from  the  bottom,  of  red,  green,  blue,  and 
violet.  If  the  slit  be  then  made  a  little  wider,  the  patches  at 
their  edges  overlap  each  other,  and,  as  a  painter  would  say, 
produce  by  the  mixture  of  their  elementary  colours  various 
new  tints.  Then  the  spectrum  consists  of  the  seven  colours 
commonly  enumerated  and  seen  in  the  rainbow,  viz.  red,  yel- 
low, orange,  green,  blue,  indigo,  and  violet.  Had  red,  yel- 
low, blue,  and  violet  been  the  four  colours  obtained  in  the  first 
experiment,  the  occurrence  of  the  others,  viz.  of  the  orange, 


COLOURED  REFRACTION.  143 

from  the  mixing  edges  of  the  red  and  yellow — of  the  green, 
from  the  mixture  of  the  yellow  and  blue, — and  of  the  indigo, 
from  the  mixture  of  blue  and  violet,  would  have  been  antici- 
pated.   But  the  true  facts  of  the  case  not  being  such,  prove  that 
they  are  not  yet  well  understood.     When  Newton  first  made 
known  the  phenomenon  of  the  many-coloured  spectrum,  and 
the  extraordinary  conclusions  to  which  it  led,  he  excited  uni- 
versal astonishment;  for  the  common  idea  of  purity,  the  most 
unmixed,  was  that  of  white  light.     In  farther  corroboration  of 
the  notion  of  the  compound  nature  of  light,  he  mentioned,  that 
if  the  colours  which  appear  on  the  spectrum  be  painted  sepa- 
rately round  the  rim  of  a  wheel,  and  the  wheel  be  then  turned 
rapidly,  the  individual  colours  cease  to  be  distinguished,  and  a 
white  band  only  appears  where  they  are  whirling:  also,  that  if 
the  rays  of  the  spectrum,  produced  by  a  prism,  be  again  ga- 
thered together  by  a  lens,  they  reproduce  white  light.     The 
red  is  the  kind  of  light  which  is  least  bent  in  refraction,  and 
the  violet  that  which  is  most  bent.     It  was  at  one  time  said, 
as  an  explanation,  that  the  differently  coloured  particles  in 
light  had  different  degrees  of  gravity  or  inertia,  and  were,  . 
therefore,  not  all  equally  bent.     It  is  farther  remarkable,  with 
respect  to  the  solar  spectrum,  that  much  of  the  heat  in  the  ray 
is  still  less  refracted  than  even  the  red  light,  for  a  thermome- 
ter held  below  the  red  light  rises  higher  than  in  any  part  of 
the  visible  spectrum; — and  that  there  is  an  influence  more  re- 
frangible than  even  the  violet  rays,  producing  powerful  che- 
mical and  magnetical  effects.     The  different  spots  of  colour 
are  not  all  of  the  same  size,  and  there  is  a  difference  in  this  re- 
spect according  to  the  refracting  substance. 

All  transparent  substances,  in  bending  light,  produce  more  or 
less  of  the  separation  of  colour;  but  it  is  important  to  remark, 
that  the  quality  of  merely  bending  a  beam,  or  of  refraction, 
and  that  of  dividing  it  into  coloured  beams,  or  of  dispersion, 
are  distinct  qualities,  and  by  no  means  proportioned  to  each 
other  in  the  same  substances.  Newton,  from  not  discovering 
this,  despaired  that  a  perfect  telescope  of  refraction  could  ever 
be  made:  he  supposed  that  the  bent  light  would  always  become 
coloured,  and  so  render  the  objects  indistinct.  We  now  know, 


144  LIGHT. 

however,  that  by  combining  two  or  more  media,  we  may  ob- 
tain bending  of  light  without  dispersion, — thus,  by  opposing 
a  glass  which  bends  five  degrees  and  disperses  one  degree,  to 
another  glass  which  bends  three  degrees  and  disperses  one,  the 
opposing  dispersions  will  just  counterbalance  or  neutralize  each 
other,  while  the  two  degrees  of  excess  of  bending  will  remain 
to  be  applied  to  use. 

The  diversified  colours  of  the  substances  around  us  depend 
merely  upon  the  fitness  of  these,  from  texture  or  other  cause,  to 
reflect  or  transmit  certain  modifications  of  common  light,  and 
the  colour  is  not  a  part  or  property  of  the  body  itself.  We 
shall  soon  find  that  the  vivid  colours  of  the  rainbow  are  mere- 
ly the  white  light  of  the  sun:  reflected  to  us  after  being  bent 
and  modified  by  the  colourless  drops  of  falling  rain;  and  that 
the  sparkling  with  appearance  of  rubies  and  emeralds,  which 
we  see  in  the  cut-glass  lustre,  is  a  phenomenon  of  the  same 
kind: — and  that  by  scratching  the  surface  of  a  piece  of  metal 
so  as  to  have  a  given  number  of  lines  in  a  given  space,  we  can 
cause  the  same  substance  to  appear  of  any  colour  we  please. 

"  Transparent  bodies,  as  glass ,  may  be  made  of  such  form 
as  to  cause  all  the  rays  of  light  which  pass  through  them 
from  any  one  point,  to  bend  so  as  to  meet  again  in  ano- 
ther point  beyond  them, — the  body  itself,  from  the  re- 
quired form  generally  resembling  that  of  a  flat  bean  or 
lentil,  being  then  called  a  "LENS."  (Read  the  Analysis, 
page  122.) 

The  innumerable  rays  of  light,  of  which  five  only  are  here 
represented,  issuing  from  any  point  as  c,  towards  any  surface 
in  the  situation  a  b,  are  said  to  form  a  cone  or  pencil  of  di- 
verging light.  Now,  it  is  evident  that  to  make  all  such  rays 
converge  or  meet  again  in  one  place,  as  f,  beyond  a  transpa- 
rent body  placed  at  a  b,  it  would  be  necessary,  while  the  mid- 
dle ray  or  axis  of  the  pencil,  c  d,  did  not  bend  at  all,  for  the 
others  to  be  bent  more  and  more,  in  proportion  as  they  fell 
upon  the  body  farther  and  farther  from  the  centre  d.  Recol- 
lecting then  the  law  of  refraction,  that  light  entering  from  air 
through  the  surface  of  any  denser  medium,  as  glass,  is  bent 


REFRACTION  BY  LENSES.  145 


there  towards  the  perpendicular  at  the  internal  surface,  in  pro- 
portion to  the  obliquity  of  incidence,  and  on  leaving  the  op- 
posite surface,  is  correspondingly  bent  away  from  its  external 
perpendicular,  (see  the  case  of  the  prism  at  p.  141,)  we  see 
that  if  a  piece  of  glass  were  placed  at  «  b,  of  such  form  that 
the  rays  falling  upon  it  from  c  should  meet  and  leave  its  sur- 
faces with  greater  and  greater  obliquity  in  some  regular  pro- 
portion, as  the  points  of  incidence  were  more  distant  from  the 
centre  d,  the  purpose  would  be  obtained.  And  we  have  the 
pleasure  of  knowing  that  a  glass,  of  which  the  surface  is 
ground — which  it  easily  may  be — to  have  a  regular  convexity 
or  bulging,  as  if  it  were  a  portion  cut  off  from  the  surface  of 
a  globe,  can  be  shown  to  answer  very  correctly  the  required 
condition.  Such  a  glass,  similarly  ground  on  both  sides,  is 
here  represented  edgeways,  between  «  and  &,  where  the  ray,  c  d, 
falling  on  its  middle,  or  perpendicularly,  and  similarly  leaving 
it,  is  seen  going  straight  through  to  f;  but  the  ray,  c  e,  meet- 
ing the  surface  with  a  certain  degree  of  obliquity,  is  bent  down 
a  little,  first  on  entering  the  surface  at  e,  and  then  as  much 
more  on  leaving  the  opposite  surface  with  equal  obliquity,  and 
"so  arrives  at  f;  then  the  ray  c  a,  for  corresponding  reasons,  is 
still  more  bent,  and  equally  arrives  at  f; — and  the  case  would 
be  similar  of  any  other  rays  that  might  be  examined.  The 
point  f  is  usually  called  a  focus,  (meaning  a  fire  place,)  be- 
cause when  the  light  of  the  sun  is  thus  gathered,  the  heat  con 
centrated  with  it  is  powerful  enough  to  make  combustibles  in- 
flame. We  have  here  to  remark  farther,  that,  in  accordance 
both  with  calculation  and  experiment,  the  direction  in  which 

19 


146  LIGHT. 

s 

a  pencil  of  rays  falls  upon  a  lens  does  not  affect  the  result  of 
the  convergence  to  a  focus,  only  the  focus  is  always  in  the  di- 
rection of  the  central  ray  of  the  pencil  or  beam;  it  will  be  at 
p,  for  instance,  for  light  issuing  from  o,  and  at  z  for  light  is- 
suing from  x. 

The  lens  represented  at  a  b  above,  or  at  fig.  1,  in  the  annexed 
diagram,  having  both  sides  convex,  is  called  a  double  convex 
lens.  A  glass  convex  only  on  one  side,  and  plane  or  flat  on 
Fig.  1234  5  6  the  other,  as  shown 

at  (fig  2,)  would  as 
effectually     gather 
the  rays,  but  with 
half    the     power, 
and   the   point    of 
meeting  or  focus  would  be  therefore  so  much  the  more  distant. 
Such  a  glass  is  called  a  plano-convex  lens.     Then  the  gather- 
ing or  converging  power  of  any  glass,  whether  doubly  or  singly 
convex,   is  in   proportion  to   the  degree  of    its  convexity   or 
bulging  of  surfaces;  for  the  less  it  bulges,  the  more  nearly  does 
it  approach  to  a  plane  glass,  and  the  more  it  bulges,  the  more  ob- 
liquely will  the  rays  at  any  distance  from  the  centre  fall  upon  its 
surface,  and  the  sooner,  therefore,  in  consequence  of  their  being 
more  bent,  will  they  all  meet  the  axis-ray; — hence  fig.  1  would 
converge  much  more  quickly  than  fig.  3,  which  represents  near- 
ly a  common  spectacle  glass;  and  a  very  minute  globe  is  the 
form  most  powerfully  converging  of  all.     The  surfaces  of  fig. 
1,  are  portions  of  a  comparatively  small  globe;  those  of  fig.  3 
are  comparatively  smaller  portions,  but  of  a  globe  much  larger. 
Concave  lenses  as — fig.  4,  a  double  concave,  and  fig.  5,  a  plano- 
concave lens,  in  obedience  to  the  same  law  of  refraction,  spread 
rays,  or  bend  them  away  from  the  axis  of  the  pencil,  in  the 
same  degree  that  similarly  convex  lenses  gather  them.     A  con- 
cave lens,  therefore,  receiving  the  converging  pencil  of   rays 
from  a  convex  lens,  might  restore  them  to  their  former  direc- 
tion.    Very  useful  purposes,  as  will  be  afterwards  explained, 
are  served  in  optics,  by  certain    combinations   of   differently 
formed  lenses.     A  lens  may  be  convex  on  one  side  and  con- 
cave on  the  other,  as  at  fig.  6,  a  meniscus  lens  (so  called  be- 


IMAGES  BY  REFRACTION.  147 

cause  it  resembles  the  crescent  moon,)  and  its  effect  will  be  ac- 
cording to  the  form  which  predominates. 

A  person  recollecting  the  case  of  the  <f  multiplying  glass," 
described  a  few  pages  back,  might  say, — but  is  not  a  convex 
lens  merely  a  multiplying  glass  of  a  much  greater  number  of 
faces,  and  why  then,  instead  of  one  image,  does  it  not  make 
thousands?  The  answer  is,  that  the  multiplying  glass,  by 
every  face,  bends  a  set  of  rays,  capable  of  forming  a  distinct 
image:  but  the  lens  has  no  surface  large  enough  to  bend  more 
than  a  single  ray,  and  it  concentrates  all  the  single  rays  into 
one  place,  to  form  there  one  image  of  great  vividness  and 
beauty. 

"  Jlnd  when]  the  light  proceeding  from  every  point  of  an 
object  placed  before  a  lens  is  collected  in  corresponding 
points  behind  it,  a  perfect  image  of  the  object  is  there 
produced.  When  the  image  is  received  upon  a  suitable 
white  surface,  in  a  dark  place,  the  arrangement  is  called 
according  to  minor  circumstances,  the  CAMERA  OBSCURA, 
SOLAR  MICROSCOPE,  or  MAGIC  LANTERN.'*  (Read  the  Ana- 
lysis, page  122.) 

Words  are  wanting  to  express  the  admirable  consequences  to 
man  of  the  curious  property  of  a  lens,  that  it  can  bring  together 
to  a  focal  point  all  the  rays  of  light  which  traverse  it  from  any 
one  point  of  an  object  placed  before  it.  The  following  instance 

will  lead  to  the  consideration 
and  understanding  of   others. 

I  If  a  lens,  as  a,  be  placed  so  as 

-V; .-  ...  to  fill  up  an  opening  made  in 

the  window  shutter  of  a  dark- 
ened room,  then  from  any  object  before  that  opening,  as  the 
cross  here  represented,  all  the  light  which  each  point  of  the 
cross  emits  towards  the  lens  will  be  concentrated  or  gathered 
together  in  a  corresponding  focal  point  behind  the  lens  or  with- 
in the  room,  and  if  a  sheet  of  paper  be  held  there  at  the  dis- 
tance of  the  focal  points,  a  beautiful  image  of  the  object  will 
be  seen  upon  the  paper. 

In  these  few  words,  we  have  described  the  interesting  contri- 


148  LIGHT. 

vance  called  the  camera  obscura  or  dark  chamber;  and  when 
a  glass  is  chosen  of  proper  size  and  focal  distance,  and  a  screen 
or  the  wall  of  the  chamber  is  properly  prepared  to  receive  the 
light,  the  most  enchanting  portraiture  is  instantly  produced  of 
the  whole  scene  which  the  window  commands.  With  what 
rapture  does  the  school  boy  first  view  this  lovely  picture  drawn 
by  nature's  own  pencil,  and  with  colours  borrowed  directly 
from  the  sun's  bright  ray — with  what  rapture,  as  his  eyes  search 
over  it,  does  he  recognise  his  playmates  there,  and  perhaps  the 
river  in  which  he  bathes,  and  where  he  sails  his  boat,  and  the 
wood  in  whose  solitudes  he  loves  to  wander,  and  the  mountain 
heights  which  he  climbs  to  meet  the  fresh  breeze,  and  at  a  dis- 
tance from  the  world,  to  allow  play  to  the  workings  of  his 
young  fancy,  beginning  to  shoot  far  into  time  and  space.  The 
great  peculiarity  of  such  a  picture  is,  that  it  does  not  like  others 
portray  still  nature,  but  every  thing  with  appropriate  motion 
or  changes:  there  the  playmates  are  all  in  action;  the  leafy  trees 
may  wave  in  the  wind,  the  clouds  may  sail  along,  the  sun  may 
rise  or  may  set,  and  even  the  lightning's  gleam  may  dart  across: 
or,  again,  commenced  enterprises  may  be  brought  to  a  close, 
the  traveller  may  climb  the  distant  hill  and  disappear,  the  fish- 
erman may  draw  his  net  and  store  his  gains,  the  well  contested 
race  may  be  won  or  lost.  A  Malayan  chief  in  the  island  of 
Sumatra,  was  so  surprised  and  pleased  by  a  small  portable  ca- 
mera obscura  which  the  author  had  among  his  apparatus,  that 
he  seemed  disposed  to  give  for  it  almost  any  thing  he  pos- 
sessed. 

It  appears  in  the  last  diagram  that  the  image  formed  beyond 
a  lens  by  the  gathered  light,  is  in  a  contrary  position  to  the  ob- 
ject itself, — that  is,  inverted, — because,  the  light  from  the  top 
of  the  object,  darts  through  the  opening  or  glass  in  a  descend- 
ing direction,  and  that  from  the  bottom  rises  to  the  opening, 
and  in  the  same  direction  passes  beyond  it.  It  is  usual,  there- 
fore, in  a  camera  obscura  to  place  a  small  mirror  immediately 
behind  the  lens,  so  as  to  throw  all  the  light  which  enters, 
downwards  to  a  whitened  table,  upon  which  the  picture  may 
be  conveniently  contemplated. 

The  camera  obscura   often  gives  very  useful   assistance  to 


CAMERA  OBSCURA.  149 

young  painters,  by  enabling  them  to  trace  correctly  the  outlines 
of  the  objects  placed  before  it,  and  also  to  study  effects  of  light, 
shade,  and  colour,  more  profitably  than  they  at  first  can,  by 
looking  at  the  objects  themselves.  The  laws  of  perspective 
are  most  intelligibly  illustrated  in  this  most  true  picture. 

An  effect  approaching  in  a  degree  to  that  of  the  complete 
camera  obscura  now  described,  is  produced  by  merely  making 
a  small  hole  in  the  shutter  of  a  dark  room,  and  letting  the  light 
which  enters  by  it  fall  on  any  white  surface  beyond.  The 
whole  landscape  is  then  dimly  portrayed  upon  the  surface.  If 
a  cross  be  held  before  the  opening  as  in  the  last  figure,  it  is  evi- 
dent that  from  every  point  of  the  cross  light  will  enter  by  the 
opening,  and  will  fall  on  corresponding  parts  of  a  sheet  of  pa- 
per held  behind, — but  as  the  light  from  each  point  spreads  as 
it  departs,  becoming  a  pencil  or  cone  of  light  instead  of  a  ray, 
it  will  fall  on  a  surface  of  the  paper  at  least  as  large  as  the 
opening,  and  thus  the  light  from  adjoining  spots  will  mix  at 
the  edges,  and  will  render  the  images  misty  and  indistinct, 
somewhat  like  those  on  the  back  of  tapestry.  If  the  opening 
be  very  small,  the  picture  will  be  well  defined,  but  very  feebly 
illuminated;  and  if  the  opening  be  of  considerable  size,  the 
mixing  of  the  pencils  will  be  so  great  as  to  leave  no  particular 
object  distinguishable.  In  the  latter  case,  and  supposing  the 
opening  to  be  larger  than  the  pupils  of  a  thousand  eyes,  if  a 
lens  be  introduced,  it  will  converge  every  pencil  of  light  to  an 
exact  point,  and  the  picture  will  instantly  be  rendered  perfectly 
clear. 

The  distance  from  a  lens  at  which  an  image  is  formed  or  the 
rays  of  the  light  meet,  depends,  first,  upon  the  refractive  or 
bending  power  of  the  lens,  and  therefore  on  the  nature  of  its 
substance,  and  the  form  of  the  lens;  and,  secondly,  upon  the 
direction  of  the  rays  of  light  when  they  reach  the  lens,  viz. 
whether  they  are  divergent,  parallel,  or  convergent.  We  have 
already  explained  that  glass  refracts  about  twice  as  much  as  wa- 
ter, and  that  diamond  refracts  about  twice  as  much  as  glass; 
and  we  have  considered  the  effect  of  different  degrees  of  con- 
vexity in  lenses — arising  equally  whether  the  lens  be  of  water 
enclosed  between  glasses  like  watch  glasses,  or  of  solid  glass, 


150  LIGHT 

or  of  rock  crystal,  or  of  diamond  itself.  We  now  proceed  to 
consider  the  joint  effect  of  the  refractive  power,  and  of  the  di- 
rection of  the  incident  rays. 

Rays  falling  from  a  on  a  comparatively  flat  or  weak  lens  at 
L,  might  meet  only  at  d,  or  even   farther  off;  while  with  a 


stronger  or  more  convex  lens,  they  might  meet  at  c  or  at  b:  a 
lens  weaker  still  might  only  destroy  the  divergence  of  the  rays, 
without  being  able  to  give  them  any  convergence,  or  to  bend 
them  enough  to  bring  them  to  a  point  at  all, — and  then  they 
would  proceed  all  parallel  to  each  other,  as  seen  at  e  and  f: — 
and  if  the  lens  were  yet  weaker,  it  might  only  destroy  a  part 
of  the  divergence,  causing  the  rays  from  a  to  go  to  g  and  A, 
after  passing  through,  instead  of  to,  i  and  k,  in  their  original 
direction. 

In  an  analogous  manner,  light  coming  to  the  lens,  in  the  con- 
trary directions  from  b  c  d^  &c.,  might,  according  to  the  strength 
of  the  lens,  be  all  made  to  come  to  a  focus  at  a  or  at  /,  or  in 
some  more  distant  point;  or  the  rays  might  become  parallel, 
as  m  and  n,  and  therefore  never  come  to  a  focus,  or  they  might 
remain  divergent. 

It  may  be  observed  in  the  figure  above,  that  the  farther  aa 
object  is  from  the  lens,  the  less  divergent  are  the  rays  darting 
from  it  towards  the  lens;  or  the  more  nearly  do  they  approach 
to  being  parallel.  From  b  there  is  much  divergence,  from  c 
less,  from  d  less  still,  and  rays  from  a  great  distance,  as  those 
cut  off  at  efj  appear  quite  parallel.  If  the  distance  of  the  ra- 
diant point  be  very  great,  they  really  are  so  nearly  parallel  that 
a  very  nice  test  is  required  to  detect  the  non-accordance.  Rays, 
for  instance,  coming  to  the  earth  from  the  sun,  do  not  diverge 
the  millionth  of  an  inch  in  a  thousand  miles.  Hence  where  we 


CAMERA  OBSCURA.  151 

wish  to  make  experiments  with  parallel  rays,  we  take  those  of 
the  sun. 

Any  two  points  so  situated  on  the  opposite  sides  of  a  lens, 
as  that  wheh  either  becomes  the  radiant  point  of  light,  the  other 
is  the  focus  of  such  light,  are  called  conjugate  foci.  An  ob- 
ject and  its  image  formed  by  a  lens  must  always  be  in  conju- 
gate foci,  and  when  the  one  is  nearer  the  lens,  the  other  will 
be  in  a  certain  proportion  more  distant. 

What  is  called  the  principal  focus  of  a  lens,  and  by  the  dis- 
tance of  which  from  the  glass  we  compare  or  classify  lenses 
among  themselves,  is  the  point  at  which  the  sun's  rays  are  made 
to  meet;  and  thus,  by  holding  the  glass  in  the  sun,  and  noting 
at  what  distance  behind  it  the  little  luminous  spot  or  image  of 
the  sun  is  formed,  we  can  at  once  ascertain  the  focus  of  a  glass* 
as  at  a  for  the  rays  e,  and  f. 

It  is  remarkable  that  the  bending  power  of  the  common  glass 
used  for  lenses  should  be  such,  that  the  focus  of  a  double  lens  of 
glass  is  just  where  the  centre  of  the  sphere  would  be,,  of  which 
the  surface  of  the  lens  is  a  portion.  This  gives  us  another  fact 
with  which  to  associate  the  recollection  that  the  focus  is  near  as 
the  convexity  of  the  lens  is  greater,  that  is  to  say,  as  the  surface 
is  a  portion  of  a  smaller  sphere.  And  such  being  the  law,  it  may 
be  proved  by  calculation,  as  well  as  by  the  fact,  that  if  a  candle 
be  held  from  a  lens  at  twice  the  principal  focal  distance,  suppose 
at  c  for  a  lens  with  the  focus  at  cr,  the  image  of  the  candle  will 
be  formed  at  /just  as  far  on  the  other  side.  Thus,  then,  by  try- 
ing with  a  lens  until  the  image  of  a  candle  is  formed  at  the  same 
distance  from  it  as  the  object  is,  we  have  a  second  mode  of  as- 
certaining the  focal  distance  of  a  lens.  Other  kinds  of  glass  and 
Other  substances  refract  with  different  power;  but  the  facts  now 
stated  should  be  retained  in  the  memory  as  standards  of  com- 
parison. 

Because  the  focal  point  of  light  passing  through  a  lens  is  at  the 
same  distance  from  the  centre  of  the  lens,  in  whatever  direction 
the  light  passes  through,  a  surface  placed  to  receive  the  image 
of  any  object,  should  really  be  concave;  that  is  to  say,  all  parts 
of  it  should  be  at  the  same  distance  from  the  centre  of  the  lens, 
otherwise  the  image  will  be  more  perfect  either  at  its  middle  than 


152  LIGHT. 

towards  its  edges,  or  vice  versa — but  it  is  not  found  necessary 
to  attend  to  this  in  common  practice. 

The  size  of  an  image  formed  behind  a  lens  is  always  propor- 
tioned to  its  distance  from  the  lens,  and  the  image  is  as  much 
larger  or  smaller  than  the  object  as  it  is  farther  from  or  nearer  to 
the  lens  than  the  object.  This  will  be  evident  from  considering 

the  annexed  figure,  c  repre- 
sents a  lens,  which,  according 
to  its  power,  will  form  an  image 
of  the  cross  a  b  in  some  situ- 
ation, as  at  d,  e,  g,  &c.  Now 
wherever  the  image  is  formed, 

and  by  whatever  lens,  one  end  of  it  must  be  in  contact  with  the 
line  a  g,  and  the  other  end  with  the  line  b  h;  and  as  these  lines 
cross  each  other  at  c,  and  widen  regularly  afterwards,  a  line  join- 
ing them  (and  the  image  is  really  such  a  line,)  must  always 
be  shorter  the  nearer  it  is  to  c;  that  is  to  say,  shorter  in  propor- 
tion to  the  converging  power  of  the  lens. 

Many  persons  may  not  have  reflected,  that  the  little  luminous 
circle  called  the  focus  of  a  burning  glass,  is  really  but  the  image 
or  picture  of  the  sun  formed  by  that  glass  or  lens.  The  inten- 
sity of  the  heat  and  of  the  light  is  of  course  in  proportion  as 
the  image  is  smaller  than  the  glass  which  forms  it,  and  the  near- 
er that  the  image  is  formed  to  the  lens,  or  the  more  powerfully 
convergent  that  the  lens  is,  the  smaller  will  the  image  be.  Mr. 
Parker's  famous  burning  lens,  which  cost  ;£700,  and  is  now  the 
property  of  the  emperor  of  China,  was  three  feet  in  diameter, 
and  the  diameter  of  the  sun's  image  formed  by  it  was  one  inch:  it 
concentrated,  therefore,  about  1,300  times  to  render  the  effect 
still  more  powerful,  a  smaller  lens  was  placed  behind  the  larger, 
farther  reducing  the  size  of  the  image  to  one-sixth.  Very  sur- 
prising effects  were  produced  by  this  lens,  in  the  melting  of  me- 
tals, inflaming  of  combustibles,  &c.  The  size  of  burning  lenses, 
until  lately,  was  limited  by  the  difficulty  of  obtaining  the  great 
pieces  of  glass  required  to  form  them:  but  they  are  now  built 
up  of  many  pieces  suitably  united  together.  Some  large  lenses 
have  been  made  of  water,  that  is,  of  water  enclosed  between 
meniscus  glasses,  like  watch  glasses.  A  common  goblet  of  wa- 


SOLAR  MICROSCOPE.  153 

ter,  or  a  vase  holding  gold  fishes,  has  acted  as  a  burning  glass  in 
some  cases,  where,  in  consequence  of  its  being  left  in  a  sunny 
window  near  the  curtains,  a  house  has  been  burned. 

And  the  nearer  that  an  object  is  brought  to  a  lens,  the  more 
distant,  and  therefore  the  larger  will  its  image  be:  for,  as  the 
rays  towards  a  lens  diverge  in  proportion  to  the  nearness  of  the 
object,  and  therefore  with  the  same  power  of  lens,  must  meet 
farther  behind, — as  seen  in  the  figure  at  page  201,  then  the  axis 
of  the  rays,  as  the  lines  c  a  and  c  b  in  the  last  figure,  will  have 
separated  far  before  the  rays  meet,  and  will  have  made  the  image 
proportionally  larger.  If  we  suppose  little  d  in  the  same  dia- 
gram to  be  the  object,  its  image  would  be  a  b.  The  sun  is  ex- 
actly as  much  larger  than  his  image  formed  by  a  burning  glass, 
as  he  is  more  distant  from  it  than  the  image;  and  if  we  had  a 
canvas  of  sufficient  size  hung  up  in  a  distant  space,  a  very  bright 
object  of  a  quarter  of  an  inch  diameter  might  be  made  to  form 
an  image  as  broad  as  the  sun. 

From  all  these  considerations,  we  see  that,  in  a  camera  obscu- 
ra,  the  screen  should  be  from  the  lens,  at  the  distance  of  its  prin- 
cipal focus  for  distant  objects,  and  a  little  farther  than  this  for  near 
objects.  Accordingly,  the  lens  is  generally  fixed  in  a  sliding 
piece,  which  allows  the  distance  from  the  screen  to  be  adjusted  to 
circumstances.  If  therepresentatipn  is  wished  to  be  large,  the  lens 
must  be  of  a  long  focus;  if  to  be  small,  the  lens  must  be  of  a  short 
focus.  Again,  when  by  the  reversed  use  of  the  lens,  a  small  object, 
as  d,  is  to  be  magnified  to  such  a  size  as  a  b,  then  the  object  must 
be  placed  a  little  beyond  the  focus  of  the  glass;  for  if  placed  near- 
er, the  pencils  of  rays  from  it  would  never  be  gathered  to  fo- 
cal points,  and  no  image  would  be  formed  at  any  distance. 

When,  as  alluded  to  in  the  last  sentence,  a  small  object  is 
placed  very  near  a  lens,  and  the  image  of  it  is  thrown  upon  the 
wall  of  a  dark  room,  perhaps  a  hundred  times  farther  from. the 
lens  than  the  object  is,  the  image  is  a  greatly  magnified  repre- 
sentation of  the  object,  viz.  it  is  a  hundred  times  longer  and  a 
hundred  times  broader,  and  therefore  has  ten  thousand  times  as 
much  surface  as  the  object;  but  if  in  this  experiment  the  object 
be  illuminated  only  in  an  ordinary  degree,  the  light  from  it  is 
so  scattered  as  not  to  suffice  for  distinct  vision.  Hence,  to  at- 

20 


154  LIGHT. 

tain  fully  in  this  manner  the  purpose  of  a  microscope,  a  very 
strong  light,  concentrated  by  a  suitable  mirror  or  glass,  must  be 
directed  upon  the  object.  When  the  light  of  the  sun  is  used 
in  such  a  case,  the  complete  apparatus  is  called  the  solar  micro- 
scope, and  serves  beautifully  to  display  the  structure  of  many 
minute  objects.  When  artificial  light  is  used,  as  of  a  lamp,  the 
apparatus  is  called  the  lucernal  microscope  or  magic  lantern. 
A  good  solar  microscope  becomes  one  of  the  most  interest- 
ing presents  which  science  has  made  to  man,  for  aiding  him  in 
his  researches  into  the  secrets  of  nature.  With  the  late  im- 
provements in  the  construction  of  lenses,  by  which  the  disper- 
sion of  light,  or  the  rainbow  fringe,  is  prevented  (as  will  be 
explained  under  the  head  of  "  Tejescopes/')  objects  maybe 
magnified  two  or  three  hundred  thousand  times,  and  still  be  so 
luminous  as  to  be  beautifully  distinct: — thus  a  cheese-mite  will 
appear  of  the  dimensions  of  a  little  pig,  and  creatures  altogether 
invisible  to  the  naked  eye,  or  perceived  by  it  only  as  minute 
white  points,  are  discovered  to  be  animated  beings,  having  the 
perfect  proportions,  and  often  the  great  beauty  of  larger  ani- 
mals, and  endowed  with  similar  appetites,  passions,  and  appa- 
rent ingenuity,  but  with  an  activity  far  surpassing  that  met 
with  in  the  more  bulky  creation.  A  judicious  selection  of  ob- 
jects, for  the  solar  microscope,  is  calculated  exceedingly  to 
surprise  the  mind  on  first  attending  to  them,  and  to  fill  it  with 
high  conceptions  of  the  infinity  of  God's  creation.  With  the 
common  microscope  only  one  person  at  a  time  can  feast  his 
wonder;  but  with  the  solar,  a  whole  roomful  of  company  may 
at  once  contemplate  the  same  objects  and  witness  the  same  ac- 
tions, and  thus  have  their  admiration  increased  by  the  con- 
sciousness of  a  sympathy. 

The  magic  lantern,  we  have  said,  consists  of  a  powerful  lens, 
with  objects,  highly  illuminated  by  lamp  light,  placed  so  near 
it  that  their  imnges  are  formed  far  off,  and  are  therefore  pro- 
portionally larger.  For  the  magic  lantern  the  objects  are  ge- 
nerally paintings  on  glass  made  with  transparent  colours;  and 
the  glass  is  formed  to  slide  through  a  slit  or  passage  behind  the 
lens.  The  lens  itself, — or  what  may  be  called  half  of  it,  for 
there  are  often  two  lenses  joined  to  give  greater  power,  is 


MAGIC  LANTERN.  155 

moveable  with  the  tube  which  is  seen  projecting  from  the  lan- 
tern, so  that  its  distance  from  the  object  may  be  varied,  and 
thus  a  corresponding  -approach  to  or  receding  from  the  screen 
may  be  allowed,  which  will  produce  an  increase  or  lessening 
of  the  magnitude  of  the  visible  picture  on  the  wall. 

Some  public  lecturers  on  astronomy  and  branches  of  natural 
history  prefer  having  the  drawings  and  paintings,  required  for 
the  elucidation  of  their  subjects  made  in  miniature  upon  glass, 
to  be  magnified  afterwards  to  the  degree  desired,  and  shown 
upon  any  part  of  the  lecture  room  by  the  magic  lantern. 

A  thick  fog  or  mist  at  night  will  sometimes  reflect  the  images 
of  a  magic  lantern  so  as  to  make  them  distinctly  visible;  and 
there  are  several  cases  on  record,  where  persons,  wickedly  in- 
genious in  this  way,  have  terrified  ignorant  individuals  almost 
to  death,  by  throwing  spectres  from  a  concealed  lantern.  Some 
years  ago  a  sentinel  in  St.  James'  Park  was  thus  persuaded  that 
he  had  seen  supernatural  beings  near  him  among  the  trees. 

A  very  charming  illusion  is  produced  by  a  magic  lantern 
manoeuvred  on  one  side  of  a  thin  screen,  let  down  like  the  cur- 
tain of  a  theatre,  while  the  spectators,  not  aware  of  the  exist- 
ence of  the  screen,  are  sitting  on  the  other  side.  The  image 
may  be  first  thrown  upon  the  screen  with  the  lantern  very  near, 
and  then  it  will  be  small,  and  exceedingly  bright  if  desired, 
because  the  light  is  much  concentrated.  By  the  exhibitor 
then  gradually  receding  from  the  screen,  and  at  the  same  time 
adjusting  the  distance  of  the  lens  from  the  picture,  the  image, 
which  may  be  that  of  a  genius  flying  in  the  air,  becomes  larger, 
and  to  the  spectators  who  do  not  even  know  that  there  is  a 
screen  there,  it  will  appear  to  be  soaring  and  approaching — un- 
til at  last  the  expanded  wings  and  limbs  may  seem  hovering 
almost  over  their  heads.  An  endless  variety  of  most  ingenious 
and  beautiful  exhibitions  of  this  kind  have  been  made,  under 
the  name  of  the  phantasmagoria,  or  raising  of  spectres. 

"  And  the  EYE  itself  is,  in  fact,  but  a  small  camera  obscu- 
ra, — of  which  the  pupil  is  the  round  opening  or  window 
before  the  lens."  (Read  the  Analysis,  page  122.) 

The  Eye. — And  who  could  at  first  believe  that  in  describing 


156  LIGHT. 

the  camera  obscura,  as  we  have  now  done,  we  had  in  reality 
been  describing  that  most  interesting  of  the  objects  of  creation, 
the  living  eye  itself,  the  great  inlet  of  man's  knowledge,  that 
which  may  be  called  the  visible  dwelling  of  the  soul,  or  at  least 
the  window  of  that  dwelling — that  from  which  all  the  fire  of  pas- 
sion darts,  through  which  the  languor  of  exhaustion  is  perceived, 
in  which. life  and  thought  seem  concentrated!  Yet  the  eye  is 
nothing  but  a  simple  camera  obscura,  formed  of  the  parts  de- 
scribed above  as  essential  to  the  camera  obscura: — but  in  its 
simplicity  it  is  so  perfect,  so  unspeakably  perfect,  that  the 
searchers  after  tangible  evidences  of  the  existence  of  an  all- 
wise  and  good  Creator,  have  declared  their  willingness  to  be 
limited  to  it  alone  in  the  midst  of  millions,  as  their  one  tri- 
umphant proof.  Yv'e  shall  now  describe  it  and  its  actions. 
Keeping  present  to  us  the  idea  of  the  camera  obscura,  as  alrea- 
dy treated  of,  the  use  of  the  various  parts  of  the  eye  will  be 
declared  by  merely  enumerating  them.  This  paragraph  should 
be  perused  while  the  reader  has  the  opportunity  of  observing 
either  his  own  eye  reflected  in  a  glass,  or  the  eye  of  some  com- 
panion near  him. 

The  human  eye,  then,  is  a  globular  chamber  of  the  size  of 
a  large  walnut,  formed  externally  by  a  very  tough  membrane 
called,  from  its  hardness,  the  sclerotic  coat,  in  the  front  of 
which  there  is  one  round  opening  or  window,  named,  because 
of  its  horny  texture,  the  coniea.  The  chamber  is  lined  with 
a  finer  membrane  or  web — the  choroid,  which,  to  ensure  the 
internal  darkness  of  the  place,  is  covered  with  a  black  paint, 
the  pigment-um  nigrum.  This  lining  at  the  edge  of  the 
round  window  is  bordered  by  a  folded  drapery — the  ciliary 
processes,  hidden  from  without  by  being  behind  the  curious 
contractile  window  curtain,  the  iris,  through  the  central  open- 
ing of  which,  or  pupil,  the  light  enters.  Immediately  behind 
the  pupil  is  suspended  by  attachments  among  the  ciliary  pro- 
cesses, the  crystalline  lens,  a  double  convex  most  transparent 
body  of  considerable  hardness,  which  so  influences  the  light 
passing  through  it  from  external  objects,  as  to  form  most  per- 
fect images  of  these  objects  in  the  way  already  described,  on 
the  back  wall  of  the  eye,  over  which  the  optic  nerve,  then 


THE  EYE STIIUCTURE. 


157 


called  the  retina,  is  spread  as  a  second  lining.  The  eye  is 
maintained  in  its  globular  condition  by  a  watery  liquid,  which 
distends  its  external  coverings,  and  which  in  the  compartment 
before  the  lens,  or  the  anterior  chamber  of  the  eye,  being  per- 
fectly limpid,  is  called  the  aqueous  humour,  and  in  the  re- 
mainder or  larger  posterior  chamber,  being  enclosed  in  a 
transparent  spongy  structure,  so  as  to  acquire  somewhat  of  the 
appearance  of  melted  glass,  is  called  the  vitreous  humour. 

The  annexed  figure  represents  an  eye  of  the  common  dimen- 
sions, supposed  to  be  cut  through  the  middle,  from  above  down- 


wards.  C  is  the  outer  or  sclerotic  coat,  known  popularly, 
where  most  exposed  in  front,  as  the  white  of  the  eye.  A  is 
the  transparent  cornea  joined  to  the  edge  of  the  round  open- 
ing of  the  sclerotic:  it  is  more  bulging  than  the  sclerotic,  or 
forms  a  portion  of  a  smaller  sphere  than  the  general  eye-ball, 
so  that  while  it  may  be  truly  called  a  bow  window,  it,  or  rather 
the  convex  surface  of  its  contained  water,  is  also  a  powerful 
lens  for  acting  on  the  pencils  of  entering  light.  At  13,  and  si- 
milarly all  round  the  edge  of  the  cornea,  is  attached  the  win- 
dow curtain  or  iris,  shown  here  edgeways,  immersed  in  the 
aqueous  humour,  and  hanging  inwards  from  above  and  below 
towards  its  central  opening  or  jmpil,  through  which  the  rays 
of  light  are  passing  to  the  lens.  The  iris  has  in  its  structure 
two  sets  of  fibres,  the  circular  and  "the  radiating,  which  cross 
and  act  in  opposition  to  each  other.  When  the  circular  fibres 
contract,  the  pupil  is  lessened,  when  the  radiating  contract,  it 
is  enlarged:  and  the  changes  happen  according  to  the  intensity 
of  light  and  the  state  of  sensibility  of  the  retina, — as  may  at 


158  LIGHT. 

any  time  be  proved  by  closing  the  eye-lids  for  a  moment  to 
make  the  pupil  dilate,  and  then  opening  them  towards  a  strong 
light,  to  make  it  contract.  Behind  the  pupil  is  seen  the  lens 
D  with  its  circumference  attached  to  the  ciliary  processes  E: 
it  is  more  convex  behind  than  before.  The  disease  of  the  eye, 
called  cataract,  (from  a  Greek  word  implying  obstruction,}  is 
the  circumstance  of  the  lens  becoming  opaque,  and  the  cure  is  to 
extract  the  lens  entirely,  or  to  depress  it  to  the  bottom  of  the  eye, 
and  then  to  substitute  for  it  externally  a  powerful  artificial  lens 
or  spectacle-glass.  The  three  lines,  forming  here  the  bounda- 
ry of  the  eye,  stand  for  its  three  coats,  as  they  have  been 
called,  the  strong  sclerotic,  and  the  double  lining  of  the  cho- 
roid  and  retina.  The  figure  of  a  cross  is  represented  upon 
the  retina  as  formed  by  the  light  entering  from  the  cross  with- 

J  O  O 

out,  which  cross  has  to  appear  here  small  and  near,  although 
supposed  to  be  large  and  distant.  The  image  of  the  cross  is 
inverted,  as  explained  for  the  camera  obscura:  but  we  shall 
learn  below  that  the  perception  of  an  object  may  be  equally 
distinct  in  whatever  position  the  image  be  on  the  retina.  It 
has  been  explained  above,  that  a  lens  can  form  a  perfect  image 
of  considerable  extent  only  on  a  concave  surface,  and  the  retina 
is  such  a  surface.  The  present  diagram  farther  explains  what 
is  meant  by  the  anterior  and  posterior  chambers  of  the  eye, 
viz.  the  compartments  which  are  before  and  behind  the  crys- 
talline lens  D. 

The  nature  of  the  eye  as  a  camera  obscura  is  beautifully  ex- 
hibited by  taking  the  eye  of  a  recently  killed  bullock,  and  after 
carefully  cutting  away  or  thinning  the  outer  coat  of  it  be- 
hind, by  going  with  it  to  a  dark  place  and  directing  the  pupil 
towards  any  brightly  illuminated  objects;  then,  through  the  se- 
mi-transparent retina  left  at  the  back  of  the  eye  may  be  seen  a 
minute  but  perfect  picture  of  all  such  objects — a  picture,  there- 
fore, formed  on  the  back  of  the  little  apartment  or  camera  ob- 
scura, by  the  agency  of  the  convex  cornea  and  lens  in  front. 

Understanding  from  all  this,  that  when  a  man  is  engaged  in 
Tvhat  is  called  looking  at  an  object,  his  mind  is  in  truth  only 
taking  cognizance  of  the  picture  or  impression  made  on  his  re- 
tina, it  excites  admiration  in  us  to  think  of  the  exquisite  deli- 


THE  EYE — INVERTED  IMAGE.  159 

cacy  of  texture  arid  of  sensibility  which  the  retina  must  pos- 
sess, that  there  may  be  the  perfect  perception  which  really  oc- 
curs of  even  the  separate  parts  of  the  minute  images  there 
formed.  A  whole  printed  sheet  of  newspaper,  for  instance, 
may  be  represented  on  the  retina  on  less  surface  than  that  of  a 
finger  nail,  and  yet  not  only  shall  every  word  and  letter  he  se- 
parately perceivable,  but  even  any  imperfection  of  a  single  let- 
ter. Or,  more  wonderful  still,  when  at  night  an  eye  is  turned 
up  to  the  blue  vault  of  heaven,  there  is  portrayed  on  the  little 
concave  of  the  retina  the  boundless  concave  of  the  sky,  with 
every  object  in  its  just  proportions.  There  a  moon  in  beauti- 
ful miniature  may  be  sailing  among  her  white  edged  clouds, 
and  surrounded  by  a  thousand  twinkling  stars,  so  that  to  an 
animalcule  supposed  to  be  within  and  near  the  people,  the  reti- 
na might  appear  another  starry  firmament  with  all  its  glory. 
If  the  images  in  the  human  eye  be  thus  minute,  what  must 
they  be  in  the  little  eye  of  a  canary  bird,  or  of  another  animal 
smaller  still !  How  wonderful  are  the  works  of  nature  ! 

Because  the  images  formed  on  the  retina  are  always  inverted 
as  respects  the  position  of  the  objects  producing  them— just  as 
happens  in  a  simple  camera  obscura,  persons  have  wondered 
that  things  should  appear  upright,  or  in  their  true  situations.    The 
explanation  is  not  difficult.     It  is  known  that  a  man  with  wry 
neck  judges  as  correctly  of  the  position  of  the  objects  around 
him  as  any  other  person — never  deeming  them,  for  instance, 
inclined  or  crooked,  because  their  images  are  inclined  as  re- 
gards the  natural  perpendicular  of  his  retina;  and  that  a   bed- 
ridden person  obliged  to  keep  the  head  upon  the  pillow,  soon 
acquires  the  faculty  of  the  person  with  wry  neck:  and  that  an 
afiected  girl  inclining  her  head  while  trying  her  attitudes,  from 
much  practice  judges  of  the  manoeuvres  of  a  beau  as  conve- 
niently in  that  way  as  in  any  other;  and  that  boys  who  at  play 
bend  down  to  look  backwards  through  their  legs,  although  a 
little  puzzled  at  first,  because  the  usual  position  of  the  images 
on  the  retina  is  reversed,  soon  see  as  well  in   that  way  as  in 
any  other.     It  appears,  therefore,  that  while  the  mind  studies 
the  form,  colour,  &c.  of  external  objects  in  their  images  project- 
ed on  the  retina,  it  judges  of  their  position  by  the  direction  iu 


160  LIGHT. 

which  the  light  comes  from  them  towards  the  eye — no  more 
deeming  an  object  to  be  placed  low  because  its  image  may  be 
low  in  the  eye,  than  a  man  in  a  room  into  which  a  sun-beam 
enters  by  a  hole  in  the  window  shutter,  deems  the  sun  low  be- 
cause its  image  is  on  the  floor.  A  candle  carried  past  a  key- 
hole, throws  its  light  through  to  the  opposite  wall,  so  as  to 
cause  the  luminous  spot  there  to  move  in  a  direction  the  oppo- 
site of  that  in  which  the  candle  is  carried ;  but  a  child  is  very 
young  who  has  not  learned  to  judge  at  once  in  such  a  case,  of 
the  true  motion  of  the  candle  by  the  opposite  apparent  motion 
of  the  image.  A  boatman,  who,  being  accustomed  to  his  oar, 
can  direct  its  point  against  any  object  with  great  certainty,  has 
Ion"1  ceased  to  reflect,  that  to  move  the  point  of  the  oar  in  some 
one  direction,  his  hand  must  move  in  the  contrary  direction. 
Now  the  seeing  things  upright,  by  images  which  are  inverted, 
is  a  phenomenon  akin  to  those  which  we  have  reviewed. 

Another  question  somewhat  allied  to  the  last  is,  why,  as  we 
have  two  eyes,  and  there  is  an  image  of  any  object  placed  before 
them  formed  in  each — why  the  object  does  not  appear  to  us  to 
be  double.  In  answer  to  this,  again,  we  need  only  to  state  the 
simple  facts  of  the  case.  In  the  two  eyes  there  are  correspond- 
ing points,  such  that  when  a  similar  impression  is  made  on  both, 
the  sensation  or  vision  is  single:  but  if  the  least  disturbance  of 
the  position  occur,  the  vision  becomes  double.  And  the  eyes 
are  so  wonderfully  associated,  that  from  earliest  infancy  they 
constantly  move  in  perfect  unison.  By  slightly  pressing  a  fin- 
ger on  the  ball  of  either  eye,  so  as  to  prevent  its  following  the 
motion  of  the  other,  there  is  immediately  produced  the  double 
vision;  and  tumours  about  the  eye  often  have  the  same  effect. 
Persons  who  squint  have  always  double  vision:  but  they  ac- 
quire the  power  of  attending  to  the  sensation  in  one  eye  at  a 
time.  Animals  which  have  the  eyes  placed  on  opposite  sides 
of  the  head,  so  that  the  two  can  never  be  directed  to  the  same 
point,  must  have  in  a  more  remarkable  degree  the  faculty  of 
thus  attending  to  one  eye  at  a  time. 

The  corresponding  points  in  the  two  eyes  are  equidistant  and 
in  similar  directions  from  the  centres  of  the  retinae,  called  the 
points  of  distinct  vision,  at  which  centres  the  imaginary  lines 


THE  EYE — DOUBLE  VISIOX.  161 

named  the  axes  of  the  eyes  terminate;  and  it  is  worthy  of  re- 
mark that  these  points,  in  being  both  to  the  right  or  both  to 
the  left  of  the  centres,  must  be  one  of  them  on  the  inside  of 
the  centre  as  regards  the  nose,  and  the  other  on  the  outside. 
When  the  two  eyes  are  directed  to  any  object,  their  axes  meet 
at  it,  and  the  centres  of  the  two  retinae  are  opposite  to  it,  and 
all  the  other  points  of  the  eyes  have  perfect  mutual  correspon- 
dence as  regards  that  object,  giving  the  sensation  of  single  vi- 
sion; but  the  images  formed  at  the  same  time,  of  an  object 
nearer  to  or  farther  from  the  eye  than  the  first  supposed,  can- 
not fall  on  corresponding  points;  for  an  object  nearer  than 
where  the  axes  meet  would  have  its  images  on  the  outsides  of 
the  eyes,  and  an  object  more  distant  would  have  its  images  on 
the  insides  of  the  eyes,  and  in  either  case  the  vision  would  be 
double.  Thus,  if  a  person  hold  the  two  fore-fingers  in  a  line 
from  his  eyes,  so  that  one  may  be  more  distant  than  the  other, 
by  then  looking  at  the  nearest,  the  more  distant  will  appear 
double,  and  by  looking  at  the  more  distant,  the  nearer  will  ap- 
pear double. 

The  reason  of  the  term  "  point  of  distinct  vision,"  applied 
to  the  centre  of  the  retina,  is  discovered  at  once  by  looking 
at  a  printed  page,  and  observing  that  only  the  one  letter  to 
which  the  axis  of  the  eye  is  directed,  is  distinctly  seen;  so  that 
although  the  whole  page  be  depicted  on  the  retina  at  once,  the 
eye,  in  reading,  directs  its  centre  successively  to  every  part. 

On  examining  a  dead  eye,  the  point  of  distinct  vision  is  dis- 
tinguishable from  the  retina  around  by  being  more  transparent. 
Now  it  might  have  been  expected  that  this  point  would  have 
been  the  spot  where  the  optic  nerve  enters  the  eye:  but  in 
truth  the  optic  nerve  enters  considerably  nearer  to  the  nose  than 
the  centre  of  the  retina;  and,  very  singularly,  where  it  enters, 
the  part  is  altogether  blind  or  insensible.  Had  the  two  optic 
nerves  then  entered  at  corresponding  points  of  the  retina,  (in 
the  sense  explained  above,)  there  would  have  appeared  a  black 
spot  on  every  object  opposite  to  the  insensible  points;  but  as 
the  case  really  stands,  the  part  of  any  object  from  which  the 
light  passes  to  the  insensible  part  of  one  eye  must  be  opposite 

21 


162  LIGHT. 

to  a  sensible  part  of  the  other.  The  existence  of  the  insensible 
or  blind  spot,  where  the  nerve  of  the  eye  enters,  is  discovera- 
ble by  placing  in  a  row  three  objects — wafers,  for  instance— 
on  a  table,  with  intervals  of  about  two  inches  between  them, 
and  then  looking  with  one  eye  from  a  distance  of  about 
eight  inches  at  the  wafer  which  is  towards  the  nose; — the 
middle  wafer  will  then  be  invisible,  although  the  eye  sees  that 
on  each  side  of  it;  and  if  the  eye  be  moved  still  farther  away, 
the  middle  wafer  will  come  into  view,  and  the  external  will 
disappear.  Or,  again,  the  fact  may  be  proved  by  shutting  one 
eye  and  looking  with  the  other  at  the  nail  of  a  finger  held  be- 
fore it,  while  another  finger  is  gradually  moved  away  laterally: 
the  point  of  the  moving  finger  when  at  a  certain  distance  from 
the  other  will  disappear,  but  will  be  seen  again  when  moved 
away  still  a  little  farther. 

It  appearing  then,  from  the  explanations  now  given,  that 
there  cannot  be  perfect  sight  unless  where  a  perfect  image  is 
formed  on  the  retina;  and  the  truth  having  been  formerly  ex- 
plained, that  images  behind  any  lens  will  be  at  different  dis- 
tances from  it,  according  to  the  various  distances  of  the  objects 
in  front,  that  is  to  say,  according  as  the  pencils  of  light  which 
fall  upon  it  have  more  or  less  of  divergence  in  them,  it  follows, 
that  the  eye  in  being  able,  as  it  is,  to  see  distinctly  objects  at 
any  distance  beyond  about  five  inches,  possesses  a  power  of 
altering  the  relation  of  its  parts  to  accommodate  itself  to  the 
circumstances.  We  do  not  yet  perfectly  know  whether  it  does 
this  by  lengthening  or  changing  the  form  of  the  ball  through 
the  action  of  the  surrounding  muscles,  or  by  changing  the  place 
or  the  form  of  the  lens,  but  that  one  or  more  of  these  events 
occurs  there  can  be  no  doubt. 

Among  the  eyes  of  the  myriads  of  human  creatures,  howe- 
ver, it  happens  that  all  do  not  originally  possess  these  powers 
exactly  in  the  requisite  degree,  and  that  many  lose  them,  as 
life  advances,  from  a  natural  or  usual  decay. 

Persons  are  called  short-sighted  whose  eyes  from  too  great 
convexity  of  the  cornea  or  lens,  have  so  strong  a  bending  or 
converging  power,  that  the  rays  of  light  entering  them  are 


THE  EYE — SHORT  SIGHT LONG  SIGHT.  163 

brought  to  a  focus  before 
reaching  the  retina — at  cr, 
for  instance,  instead  of  at 
bf  so  that  the  rays,  by 
spreading  again  beyond 
the  focus,  produce  on  the  retina  that  sort  of  indistinct  image 
which  is  seen  in  a  camera  obscura  of  which  the  screen  is  too 
distant  from  the  lens.  This  defect  of  sight  obliges  the  indivi- 
dual, when  using  the  naked  eye,  to  hold  objects  very  near  it, 
that  the  consequent  greater  divergence  of  the  rays  may  be  pro- 
portioned to  the  unusual  refractive  power  of  the  eye; — or  the 
person  may  find  a  remedy  in  placing  concave  lenses  between 
the  object  and  the  eyes,  which  lenses,  by  rendering  light  from 
objects  at  a  usual  distance  more  divergent,  (as  explained  at  p. 
146,)  cause  the  perfect  images  in  the  eye  to  be  formed  farther 
from  the  lens,  and  thereby  on  the  retina  itself.  Without  con- 
cave spectacles — as  the  lenses  are  called  when  fixed  together 
in  a  frame — persons  with  the  defect  now  under  consideration 
cannot  see  distinctly  any  object  that  is  distant,  and  from  which 
the  rays,  because  coming  nearly  parallel,  are  quickly  gathered 
to  a  focus.  This  defect  often  diminishes  with  years,  and  the 
person  who  in  youth  needed  spectacles,  in  old  age  sees  well 
without  them. 

There  is  an  opposite  defect  of  deficient  converging  power  in 
the  eye,  dependent  on  a  too  great  flatness  of  the  cornea  or  lens, 
which  defect  is  much  more  common  than  the  last-mentioned, 
for  the  great  majority  of  persons,  after  middle  age,  sooner  or 
later,  begin  to  experience  it.  In  this  case  the  rays  of  light 
are  not  yet  collected  into  a  focus  when  they  reach  the  retina; 
they  would  only  meet  at  b,  for  instance,  instead  of  at  c,  and, 

hence,  the  image  is  indis- 
tinct, in  the  same  manner 
as  in  a  camera  obscura  of 
which  the  screen  is  held 
too  near  the  lens.  Per- 
sons suffering  this  defect  cannot,  when  using  the  naked  eye,  see 
distinctly  any  object  very  near  to  it,  because  the  gathering  or 
converging  power  of  the  eye  cannot  conquer  the  great  diver- 


164  LIGHT. 

gence  of  rays  coming  from  a  near  point;  and,  hence,  such  per- 
sons always  remove  objects  under  examination  to  a  considera- 
ble distance,  often  to  that  of  arm's  length,  so  as  to  receive 
from  them  only  the  rays  nearly  parallel.  These  persons,  in 
contradistinction  to  the  last  described,  are  called  long-sighted 
persons.  Their  defect  is  remedied  by  the  common  convex 
spectacles,  which  do  part  of  the  converging  work,  so  to  ex- 
press ourselves,  before  the  light  enters  the  eye,  leaving  undone 
only  that  which  the  eye  can  easily  accomplish.  As  this  ail- 
ment, like  the  last,  is  met  with  in  all  degrees,  it  becomes  re- 
quisite to  choose  spectacles  accordingly:  certain  curvatures  or 
strengths  have  been  numbered  as  naturally  belonging  to  diffe- 
rent ages  or  periods  of  life,  but  each  person  should  choose,  un- 
der the  direction  of  an  experienced  judge,  until  that  strength 
is  found  which  enables  him  to  read,  without  any  straining  of 
the  eye?  at  the  common  distance  of  from  twelve  to  eighteen 
inches.  We  cannot  apply  the  mind  to  this  part  of  our  subject 
without  feeling  admiration  at  what  science  has  accomplished 
for  man  in  guarding  and  improving  his  sight.  Now,  that  in 
civilized  society  the  most  common  employments  and  enjoy- 
ments of  life  are  such  as  to  require  visual  power  capable  of  dis- 
tinguishing minute  objects — letters,  for  instance — to  deprive 
old  men  of  their  spectacles,  would  be  to  condemn  many  of 
them  to  useless  inactivity  and  a  listless  blank  of  mind  for  the 
remainder  of  their  lives. 

An  eye  much  accustomed  to  examine  near  and  minute  ob- 
jects, often  loses  something  of  its  pliancy,  and  becomes  defec- 
tive when  tried  at  distant  things,  as  the  watch-maker's  eye, 
the  engraver's,  &c.  On  the  other  hand,  the  old  seaman,  whose 
eye  has  so  often  and  uninterruptedly  been  bent  on  the  distant 
horizon,  straining  to  catch  the  view  of  an  expected  sail,  or  of 
land,  has  a  power  of  discovering  distant  things  which  is  won- 
derful; but  he  often  experiences  some  deficiency  in  regard  to 
near  things. 

A  man  who  uses  his  eyes  under  water  sees  very  indistinct- 
ly, because  the  difference  of  density  between  the  two  transpa- 
rent media — water  and  the  eye,  from  the  first  of  which,  in  the 
case  supposed,  the  light  passes  to  the  second,  is  not  so  great  as 


THE  EYE — DURATION  OF  IMPRESSIONS.  165 

between  air  and  the  eye,  the  bending  or  refraction  of  the  light 
is  consequently  not  so  great.  A  man,  to  see  well  under  wa- 
ter, therefore,  requires  to  aid  the  usual  power  of  his  eyes,  by 
strong  convex  spectacles.  It  is  for  the  reason  now  explained, 
that  the  lens  of  a  fish's  eye  is  extremely  convex:  indeed  it  is 
almost  round,  as  is  every  day  seen  in  the  white  round  head 
which  issues  from  the  eye  of  a  boiled  fish — that  little  globe 
being  the  crystalline  lens  of  the  fish  coagulated  or  hardened 
like  white  of  egg  during  the  cooking. 

There  are  many  important  considerations  connected  with  the 
sensibility  of  the  retina,  which  regard  rather  the  laws  of  life 
than  of  light,  but  we  must  here  glance  at  a  few  of  them. 

Any  impression  of  light  made  upon  the  retina  lasts  for  about 
the  sixth  of  a  second.      Hence,  when   the   burning  end  of  a 
stick  is  made  to  describe  any  line  or  curve,  its  path  becomes  a 
line  of  light;  and  if  it  revolve  in  a  circle  six  times  in  a  second, 
that  circle  will  appear  to   the  eye  a  complete  circle  of  fire. 
The  polished  end  of  an  elastic  wire,  fixed  by  its  other  end   in 
a  block  of  wood,  being  made  to  vibrate,  similarly  forms  a  line 
or  curve  of  light.     A  harp  string  while  vihrating  as  it  sounds, 
appears  like  a  flat  riband.     Lightning  or  other  meteor,  darting 
across  the  sky,  although,  in  fact,  but  a  moving  luminous  point, 
is  generally  thought  of  as  a  long  line  of  light:  the  term  forked- 
lightning  has  reference  to  this  prejudice.     The  same  remark 
applies,  in  a  degree,  to  a  sky-rocket  in  its  rapid  ascent.     Two 
or  more  colours  painted  separately  on  the  rim  of  a  wheel  which 
is  made  to  turn  rapidly,  appear  to  the  eye  to  be  as  completely 
united  as  if  they  were  really  mixed: — it  has  been  already  ex- 
plained  how  patches  of  the  various  colours  of  the  rainbow 
mixed  in  this  way  form  white  light.     If  on  one  side  of  a  card 
a  little  bird  be  painted,  and  on  a  corresponding  part  of  the  other 
side  a  cage,  then,  on  making  the  card  turn  rapidly  by  twisting 
between  the  fingers  two  threads  fixed  to  its  opposite  edges,  the 
little  bird  will  appear  to  be  imprisoned  in  the  cage:  or,  again, 
if  a  pensive  Juliet  sitting  in  her  bower  occupy  one  side  of  the 
card,  and  a  longing  Romeo  the  other,  by  the  magic  turn  of  the 
threads  the  passionate  lovers  may  instantly  be  brought  toge- 


166  LIGHT. 

ther.     Dr.  Paris  displayed  taste  and  an  amiable  ingenuity,  in 
designing  this  toy  with  great  variety  of  subjects. 

A  certain  intensity  of  light  is  necessary  to  distinct  vision, 
but  the  degree  varies  with  the  previous  state  of  the  organ.  A 
person  passing  from  the  bright  day  into  a  shaded  room,  for  a 
time  may  fancy  himself  in  total  darkness;  and  to  persons  sitting 
in  the  room  and  become  accustomed  to  the  less  light  so  as  to 
see  well  with  it,  he  will  appear  to  be  almost  blind.  The  dawn 
of  morning  after  the  darkness  of  night  appears  much  brighter 
than  an  equal  degree  of  light  in  the  evening.  When,  as  the 
night  falls,  our  lamps  or  candles  are  first' introduced,  the  glare 
is  often  for  a  time  offensive:  and  the  same  feeling  is  still 
stronger  on  opening,  in  the  morning,  bed-room  window  shut- 
ters or  close-drawn  curtains.  After  the  repose  of  night,  the 
sensibility  of  the  eye  is  such  that  the  globules  of  blood  in  the 
capillary  vessels  of  the  retina  produce  the  impression  on  it  of 
little  globes  of  light  crossing  among  each  other  as  the  tortuous 
vessels  do.  To  a  prisoner  after  long  confinement  in  a  dark 
dungeon,  the  light  of  the  sun  is  almost  insupportable.  And  a 
dungeon,  which  to  common  eyes  is  utterly  dark,  still  to  its 
long-held  inmate  has  ceased  to  be  so.  There  are  various  in- 
stances in  the  records  of  the  barbarous  ages,  of  prisoners  con- 
fined for  years  in  utter  darkness,  who  at  last  could  see  and 
make  companions  of  the  mice  which  frequented  their  cells. 
The  darkness  of  a  total  eclipse  after  bright  sunshine  appears 
much  more  deep  than  that  of  midnight,  because  of  the  sudden 
contrast.  The  long  polar  night  of  mouths  ceases  to  appear 
very  dark  to  the  polar  inhabitants,  [f  an  eye  be  directed  for 
a  time  to  a  black  wafer  laid  on  a  sheet  of  white  paper,  and  af- 
terwards to  another  part  of  the  sheet,  a  portion  of  the  paper 
of  the  size  of  the  wafer  will  appear  brilliantly  illuminated:  for 
the  ordinary  degree  of  light  from  it  appears  intense  to  the  part 
of  the  eye  lately  receiving  almost  none.  An  eye  directed  long 
and  intensely  upon  any  minute  object — as  when  a  sailor  watches 
a  speck  in  the  distant  horizon,  supposed  to  be  a  ship,  or  when 
a  hunter  on  the  brown  heath  keeps  his  eye  fixed  on  some  game 
nearly  of  the  colour  of  the  heath,  or  when  an  astronomer  gazes 
long  at  a  little  star — has  the  sensibility  of  its  centre  at  last  ex- 


THE  EYE — PERCEPTION  OF  COLOUR.          167 

haustod,  and  ceases  to  perceive  the  object;  hut  on  directing  the 
axis  of  the  eye  a  little  to  one  side  of  the  object,  so  that  an 
image  may  be  formed  only  near  the  centre,  the  object  may  be 
again  perceived,  and  the  centre  in  the  mean  time  enjoying  re- 
posp,  will  recover  its  power. 

But  the  most  extraordinary  fact  connected  with  the  sensibili- 
ty of  the  retina  is,  that  if  part  of  it  be  strongly  exercised  by 
looking  for  a  time  at  an  object  of  any  bright  colour,  on  then 
turning  the  eye  away  or  altogether  shutting  it,  an  impression 
or  spectrum  will  remain  of  the  same  form  as  the  object  lately 
contemplated,  but  of  a  perfectly  different  colour.  Thus  if  an 
eye  be  directed  for  a  time  to  a  red  wafer  laid  on  white  paper, 
and  be  then  shut  or  turned  to  another  part  of  the  paper,  a  beau- 
tifully bright  green  wafer  will  be  seen;  and  vice  versa,  a  green 
wafer  will  produce  a  red  spectrum,  an  orange  wafer  will  similar- 
ly produce  a  blue  spectrum,  a  yellow  one  a  violet  spectrum, 
&c. ;  and  a  cluster  of  wafers  will  produce  a  similar  cluster  of 
opposite  colours.  If  the  hand  be  th"n  held  over  the  eyelids  to 
darken  the  eyes  and  prevent  entirely  the  approach  of  light,  the 
spectrum  of  the  bright  parts  will  be  luminous  surrounded  by  a 
dark  ground,  and  when  the  hand  is  again  removed  the  contra- 
trary  will  be  true.  Again,  if  the  eye  be  in  a  degree  fatigued 
by  looking  at  the  setting  sun,  or  even  at  a  window  with  a 
bright  sky  beyond  it,  or  at  any  very  bright  object,  on  then  shut- 
ting it,  the  lately  contemplated  forms  will  be  perceived,  first 
of  one  vivid  colour,  and  then  of  another,  until  perhaps  all  the 
primary  colours  have  passed  in  review.  These  extraordinary 
facts  prove  that  the  sensation  of  light  and  colour,  although  ex- 
citable by  light,  is  also  producible  without  it.  This  truth  gave 
occasion  to  Darwin's  ingenious  theory,  that  the  sensation  of 
any  particular  colour,  of  red  for  instance,  is  dependent  upon  a 
certain  state  of  contraction  of  the  minute  fibres  of  the  retina, 
as  the  sensation  of  a  particular  tone  depends  on  a  certain  fre- 
quency of  vibration  of  some  part  of  the  ear,  and  that  the  fibres, 
when  fatigued  in  that  condition,  seek  relief  when  at  liberty,  by 
throwing  themselves  in  an  opposite  state, — as  a  man  whose 
back  is  fatigued  by  bending  forward,  relieves  himself  not  by 
merely  standing  erect,  but  by  bending  the  spine  backwards—* 


168  LIGHT. 

which  new  condition,  whether  produced  by  light  or  by  any 
other  cause,  gives  the  sensition  of  green.  He  applied  his  ex- 
planation similarly  to  all  other  cases  of  colour.  It  is  remarka- 
ble that  the  colours  which  thus  appear  opposite  to  each  other 
in  kmd  are  those  which  when  the  solar  spectrum  produced  by 
a  prism,  as  described  a  few  pages  back,  is  painted  round  a  wheel 
or  circle,  are  opposite  to  each  other  in  place. 

There  are  persons  who,  although  having  distinct  perceptions 
of  form  and  of  light  and  shade,  have  not  the  power  of  distin- 
guishing colours.  It  is  common  for  such  persons  to  deem  pink 
and  pea-green  (naturally  opposites)  the  same  colour,  and  there- 
fore not  to  distinguish  difference  of  colour  in  a  red  berry  and 
the  leaves  around  it.  A  man  with  this  defect,  trusting  to  his 
own  judgment,  might,  without  knowing  it,  dress  himself  like  a 
parrot. 

"  The  mind  judges  of  external  objects  by  the  relative  size, 
brightness,  colour,  fyc.,  of  the  minute  but  perfect  images 
of  them  formed  at  the  back  of  the  eye  on  the  expansion  of 
nerve  called  the  retina;  and  the  art  of  painting  is  success- 
ful in  proportion  as  it  produces  on  a  larger  scale  a  pic- 
ture* tvhich,  when  afterwards  held  before  the  eye  to  repro- 
duce itself  in  miniature  upon  the  retina,  may  excite  the 
same  impression  as  the  original  object."  (Read  the  Ana- 
lysis, page  162.) 

We  now  understand  how  an  admirable  miniature  resemblance 
of  the  objects  before  us  is  produced  upon  the  retina  of  the  eye, 
by  the  light  from  them  refracted  in  passing  through  the  different 
parts  of  the  eye;  but  after  all,  this  is  only  a  picture,  and  the  in- 
quiry remains — which  many  persons  would  suppose  so  simple 
as  to  be  trifling,  but  which  is  in  reality  most  curious  and  im- 
portant— how  we  are  thereby  enabled  to  judge  of  the  magnitudes, 
distances,  and  other  particulars  respecting  the  things  examined. 
Here  it  will  be  found,  to  the  surprise  of  persons  first  entering 
upon  the  subject,  that  we  learn  the  meaning  of  a  scene  or  picto- 
rial signs  only  gradually,  as  we  do  of  any  other  system  of  signs, 
and  that  a  person  no  more  sees,  in  the  complete  sense  of  the 
word,  that  is  to  say,  no  more  understands  any  scene  or  prospect 


THE  EYE — VISUAL  ANGLE.  169 

when  he  first  opens  his  eyes  upon  it,  and  has  a  perfect  picture 
of  it  on  his  retina,  than  he  understands  or  can  read  a  printed 
page,  on  first  looking  into  a  book  before  he  has  learned  his  let- 
ters. Most  interesting  information  has  been  obtained  on  this 
subject,  by  observing  the  facts  where  blindness  from  birth  has 
been,  by  surgical  operation,  suddenly  cured  in  persons  arrived 
at  maturity. 

If  a  man  were  placed  from  infancy  in  an  apartment  fitted  up 
as  a  camera  obscura,  and  had  no  means  of  becoming  acquainted 
with  external  nature,  but  by  watching  the  images  appearing 
upon  the  screen,  he  could  learn  almost  nothing  of  the  universe 
around  him;  but  if  after  a  time  he  were  allowed  to  walk  out,  and 
to  examine  by  the  touch  and  by  measurement  the  different  objects 
portrayed  there,  and  to  ascertain  what  size,  shape,  and  distance 
of  an  object  corresponded  with  a  certain  magnitude,  form,  po- 
sition, and  brightness  of  image,  the  imagery  might  at  last  be  to 
him  a  very  clear  indication  of  such  particulars,  and  through  them 
of  nearly  all  else  that  he  desired  to  know;  making  him  in  ima- 
gination present  to  the  objects  around,  almost  as  if  he  went  and 
examined  them  with  his  hands  like  a  blind  man,  or  in  any  other 
way.  In  the  same  manner,  nearly,  the  soul  may  be  considered 
as  if  originally  placed  in  the  little  camera  obscura  of  the  eye, 
where  it  has  to  acquire  experience  of  external  nature  by  com- 
manding the  services  of  the  bodily  limbs  or  members.  The 
judging  of  things  by  sight,  then,  is  merely  the  interpreting  one 
set  of  signs,  as  judging  by  sounds  or  language  is  interpreting 
another,  and  judging  by  hieroglyphics  or  any  written  characters 
is  interpreting  a  third.  The  common  visual  signs  on  the  retina, 
however,  are  those  most  readily  learned  or  understood,  from 
having  certain  relations  in  form,  &c.,  to  the  things  signified. 

Bodies  differ  and  are  distinguished  among  themselves  chiefly 
by  their  comparative  dimensions,  that  is,  their  size  and  shape;  and 
to  ascertain  these  and  the  relative  distances,  are  the  great  objects 
which  by  the  eye  the  mind  seeks  to  accomplish.  Now,  it  ef- 
fects its  ends  by  considering  collectively, 

1st.  The  space  and  place  occupied  by  objects  in  the  field  of 
view,  measured  by  what  is  called  the  visual  angle. 

2d.    The  intensity  of  light,  shade,  and  colour. 

22 


170 


LIGHT. 


3d.    The  divergence,  of  the  rays  of  light. 
4th.   The  convergence  of  the  axes  of  the  eyes. 
We  shall  treat  of  these  particulars  separately  in  the  order  now 
mentioned. 

1  st.  The  space  and  place  occupied  in  the  field  of  vieiv,  mea- 
sured by  the  visual  angle. 

The  field  of  view  is  that  open  or  visible  space  before  the  eyes, 
in  which  objects  are  seen;  and  the  term  may  mean  either  the 
small  field  visible  in  one  position  of  the  eyes,  or  that  which  is 
perceived  on  directing  them  all  around.  As  the  eye  may  be 
turned  in  every  direction,  it  may  be  considered  as  placed  in  the 
centre  of  a  hollow  sphere,  where  it  sees  the  several  objects  around 
occupying  certain  situations  and  certain  proportions  of  the  cir- 
cumference: and  if 
a  man  were  really 
surrounded  by  a 
large  globe  or 
sphere  of  glass,  as 
a,  through  which 
he  might  view  ob- 
jects, which  sphere 
had  any  equal  di- 
visions or  degrees 
marked  upon  it  all 
around,  he  would 
be  able  at  once  to 
say  exactly  what 
portion  of  his 
sphere  or  field  of  view  was-sfcadowed  or  occupied  by  any  sin- 
gle object,  as  the  cross  here  shown  at  z,  and  thus  to  describe 
very  intelligibly,  either  for  his  own  recollection,  or  to  inform 
others,  its  relative  magnitude  and  situation  as  then  appearing  to 
him, — just  as  he  might  say,  on  looking  at  a  tree  in  the  garden 
through  a  common  window  (which  is  a  portion  of  the  field  of 
view  really  divided  by  the  cross  bars,)  whether  he  saw  the  whole 
tree  through  one  pane  or  through  several,  and  through  which 
pane  or  panes  he  saw  it.  It  may  be  remarked  farther,  that  whe- 


THE  EYE APPARENT  SIZE  OF  OBJECTS.  171 

thcr  the  supposed  sphere  of  glass  were  large  or  small,  viz.  were 
as  a  or  as  b  or  c,  the  part  of  its  surface  apparently  occupied  by 
any  object  beyond  or  within  it,  woujd  bear  the  same  proportion 
to  the  whole  surface.  Now,  as  men  have  found  it  convenient 
to  consider  a  circle  (and  every  circle)  as  divisible  into  360  de- 
grees (which  are  smaller  therefore  in  a  small  than  in  a  large  cir- 
cle, although  in  each  having  the  safhe  relation  to  the  whole,)  the 
ready  mode  of  comparing  the  apparent  magnitude  of  objects  is 
to  say  how  many  of  these  degrees  of  the  field  of  view — supposed 
a  portion  of  a  hollow  sphere  surrounding  a  man — each  object 
occupied:  and  this  is  really  what  is  meant  by  the  apparent  size 
of  an  object.  And  because  the  most  convenient  way  of  mea- 
suring a  portion  of  a  circle,  of  which  the  whole  is  not  seen,  is 
to  measure  the  angle  formed  at  its  centre  by  lines  drawn  from 
the  extremities  of  the  portion, — as  here  the  angle  at  e  formed 
by  the  lines  c  e  and  g  e,  the  object  is  said  to  occupy  a  certain 
number  of  degrees  of  the  circumference  of  the  circle,  or  to  sub- 
tend an  angle  of  the  same  number  of  degrees  at  its  centre,  and 
this  angle  is  called  the  visual  angle,  the  subject  of  our  present 
disquisition. 

The  visual  angle  then,  in  regard  to  any  object,  is  that  in- 
cluded between  the  lines  or  rays,  as  a  u  and  d  i,  which  from 
the  extreme  points  of  the  object,  as  a  d,  cross  in  the  lens  of 
the  eye,  and  go  afterwards  to  form  the  extremes  of  the  image 
on  the  retina,  and,  as  formerly  explained,  the  angle  is  the  same 
on  either  side  of  the  lens,  viz.  towards  the  object  or  towards 
the  image.  Now,  if  all  bodies  were  at  the  same  distance  from 
the  eye,  the  magnitudes  of  their  images  formed  on  the  retina, 
or,  in  other  words,  of  the  visual  angles  subtended  by  them, 
would  be  an  exact  measure  of  their  comparative  real  magni- 
tudes, as  is 
seen  in  i  u, 
the  image  of 
the  great  cross 
a  d,  and  in  i  o, 
the  image  of 
the  small  cross 
b  d:  but  it  is  evident  here,  that  the  cross  c  c,  which  is  twice  as 


173 


LIGHT. 


large  as  b  d,  makes,  because  twice  as  far  off,  an  image  of  only 
the  same  size  as  b  d,  and  an  image,  therefore,  only  half  as  large 
as  that  of  a  cross,  a  d,  equal  in  size  with  itself:  and  the  same 
rule  of  proportion  holds  for  all  other  comparative  distances — 
at  a  hundred  times  the  distance,  an  object  appearing  only  the 
hundredth  part  as  tall,  and  so  forth.  To  judge,  therefore,  by 
the  eye  of  the  true  size  of  an  object,  we  must  know  its  dis- 
tance as  well  as  its  apparent  size  or  visual  angle. 

Many  familiar  facts  receive  their  explanation  from  the  law  of 
the  visual  angle  or  apparent  size,  being  less  always  in  pro- 
portion as  the  distance  of  an  object  is  greater. 

A  man  (or  a  cross)  at  d,  standing  near  the  outside  of  a  win- 
dow, as  b  c  (shown  here  edgeways,)  may  to  a  spectator  seated 
within  the  window  at  h,  subtend  the  same  visual  angle,  or  ap- 
pear as  tall  as  the  window,  the  light  from  his  head  passing 
through  the  top  of  the  window,  and  that  from  his  feet  passing 

through  the  bottom: 
but  if  the  man  then, 
move  away  from  the 
window,  the  specta- 
tor will  be  able  to 
see  his  whole  body 
through  a  smaller  and  a  smaller  extent  of  the  window, — as 
through  half  its  height  or  a  c,  when  he  is  twice  as  distant  from 
the  eye,  or  at  /,  and  through  the  third  or  o  c,  when  he  shall 
be  three  times  as  distant,  or  at  g,  and  so  forth,  for  any  other 
distance;  so  that  soon  a  small  figure  of  a  man  cut  in  paper,  if 
laid  upon  the  glass,  would  exactly  cover  the  part  of  it  through 
which  the  light  from  him  entered  to  the  spectator's  eye,  and 
would  then,  by  completely  hiding  him  from  view,  be  an  exact 
measure  of  his  apparent  size:  and,  at  last,  a  fly  passing  over 
the  pane  might  equally  hide  him,  and  the  fly  then  would  sub- 
tend a  larger  visual  angle  than  he,  that  is  to  say,  would  be 
forming  on  the  retina  a  larger  image  than  the  man.  Thus,  it 
often  happens  in  reality,  thaf  a  person  sitting  near  a  window, 
and  intent  upon  some  subject  of  study  or  of  conversation,  mis- 
takes a  fly  on  the  glass  for  a  man  at  a  distance \  or,  on  the  con- 


THE  EYE — APPARENT  SIZE  OP  OBJECTS.  173 

trary,  a  man  for  a  fly.  It  is  ascertained  that  the  eye,  with  an 
ordinary  degree  of  light,  can  see  an  ohject  which  in  the  field 
of  view  occupies  only  the  sixtieth  of  a  degree  (or  one  minute) 
in  a  circle  of  twelve  inches  diameter,  the  eye  being  supposed 
in  the  centre  of  the  circle.  This  space  is  about  the  one-hun- 
dredth of  any  inch  measure  held  six  inches  from  the  eye.  Now, 
a  body  smaller  than  this,  at  six  inches,  or  any  thing,  however 
large,  placed  so  far  from  the  eye  as  to  occupy  in  the  field  of 
view  less  space  than  this,  is  invisible  to  ordinary  sight.  At 
four  miles  off,  a  man  is  thus  invisible.  A  pin-head  near  will 
hide  a  house  on  a  distant  hill — nay,  will  hide  even  the  planet 
Jupiter,  although  one  hundred  thousand  times  bigger  than  this 
earth. 

In  accordance  with  the  principle  now  explained,  a  marine 
telescope  has  been  constructed,  having  the  field  of  view  di- 
vided by  fine  cross  wires,  or  otherwise,  so  that  the  person 
using  it  can  say  at  once  how  much  of  its  field  any  object  oc- 
cupies. Now,  when  ships  are  in  chase,  it  is  common  by  this 
instrument,  or  some  other  that  will  detect  a  change  of  apparent 
size,  or  of  visual  angle,  to  view  the  fleeing  or  pursuing  ship; 
and  if  the  apparent  size  be  observed  to  increase,  it  is  known 
that  the  ships  are  nearing  each  other;  if  on  the  contrary  it  di- 
minish, the  chased  ship  is  escaping. 

By  applying  this  rule,  whenever  the  exact  size  of  a  distant 
ohject  is  known  the  distance  is  ascertainable,  and,  vice  versa, 
where  the  distance  is  exactly  known,  the  size  is  determinate: 
for  it  is  evident  that  if  a  body,  as  a  ship,  known  to  be  100  feet 
tall,  occupy  or  subtend  in  the  field  of  vision  the  360th  part  of 
a  whole  circle,  or  one  degree,  the  whole  circle  must  be  360 
times  100  feet,  or  36,000;  and  knowing  the  diameter  of  such 
a  circle  to  be  nearly  one-third  as  much,  we  learn  the  distance 
of  the  ship,  viz.  half  the  diameter.  Again,  if  we  know  the 
distance  of  a  ship  or  other  object  to  be  a  mile,  and  if  we  then 
find  its  visual  angle  to  be  the  1,000th  part  of  a  circle,  we  know 
its  true  size  to  be  the  1,000th  part  of  a  circle,  of  which  the 
half-diameter  and  radius  is  one  mile.  It  is  by  applying  this 
rule  in  a  manner  to  be  afterwards  explained,  that  we  determine 
the  size  of  the  heavenly  bodies. 


1 74  LIGHT. 

We  now  perceive  that  if  the  rays  of  light  coming  to  the 
eye  through  a  plate  of  glass,  from  objects  seen  beyond  it,  could 
leave  marks  in  the  glass  at  the  points  where  they  passed,  and 
marks  capable  of  giving  out  the  same  kind  of  light  as  caused 
them,  there  would  be  formed  upon  the  glass  a  representation 
or  picture  of  the  objects  formerly  viewed  through  it,  and  that 
picture  would  be  so  perfect,  that  when  held  before  the  eye,  it 
would  form  on  the  retina  an  image  or  images  the  same  in  al- 
most all  respects  as  the  objects  themselves  had  done;  for  from 
the  different  points  of  the  glass,  light  would  dart  to  the  eye 
in  the  very  same  directions  pursued  by  that  originally  darted 
from  the  objects.  Now,  the  art  of  painting  seeks  so  to  dis- 
pose lights,  shades,  and  colours  on  any  plane  surface,  as  to  pro- 
duce the  sort  of  representation  of  objects  here  contemplated, 
while  the  picture  frame  has  to  recall  the  window  frame,  or 
edge  of  the  plate  of  glass  through  which  the  true  scene  is  sup- 
posed to  be  viewed.  It  is  remarkable,  indeed,  how  perfectly 
this  art  now  accomplishes  its  ends;  and  although  there  are  still 
trifling  differences  between  the  effect  upon  the  eye,  of  a  com- 
mon picture,  and  of  the  realities, — which  peculiarities  we  shall 
consider  presently,  and  how  they  may  be  combated  so  as  to 
render  the  illusion  quite  perfect, — it  is  not  one  of  them,  as 
might  be  supposed  from  the  small  extent  of  the  canvas,  that 
the  picture  appears  to  the  retina  smaller  than  the  objects  them- 
selves. Few  people,  before  studying  this  subject,  are  aware 
that  in  a  good  picture  the  size  of  the  figures  is  always  made  ex- 
actly such,  that  at  the  distance  from  the  eye  at  which  they  are 
meant  to  be  beheld,  they  produce  on  the  retina  the  very  same 
size  of  image  as  would  be  produced  by  the  realities  seen  under 
the  aspect  represented  in  the  picture.  To  become  sensible  of 
this,  let  a  person  look  through  a  window  pane,  with  the  eye 
at  the  distance  of  eight  inches  from  it,  and  let  him  draw  with 
a  sharp  point  upon  the  glass  previously  coated  with  gum,  the 
outline  of  the  scene  Jbeyond — perhaps  a  street  or  square,  he 
will  find,  that  the  outline  of  a  man,  seen  there  at  the  distance 
of  twenty  paces,  appearing  perfectly  to  coincide  with  the  boun- 
daries of  the  person,  and  such  as,  if  opaque,  would  just  hide 
the  person,  will  be  scarcely  half  an  inch  tall,  while  the  figure 


THE  EYE — APPARENT  SIZE  OP  OBJECTS.  175 

of  a  man  a  few  hundred  paces  off  will  appear  as  a  little  point 
too  small  for  the  minuter  features  to  be  distinguished,  even  if 
they  could  be  drawn. 

Now,  as  a  person  who  reads  the  description  of  an  elephant, 
does  not  deem  the  animal  larger  or  smaller  because  of  the  size 
of  letter  used  in  the  printing,  or  of  the  size  of  the  accompa- 
nying engraved  representation,  whether  it  be  diminutive  in 
the  page  of  a  child's  nursery  book,  or  wide  spread  over  the 
page  of  a  quarto — and  as  a  man,  viewing  in  a  picture  gallery 
miniatures  and  larger  portraits,  does  not  conceive  of  the  origi- 
nals according  to  the  size  of  the  representations — and  as  a  man 
who  views  a  picture  of  a  temple,  so  perfect  that  it  might  al- 
most be  mistaken  for  the  reality,  never  dreams,  unless  his  at- 
tention be  particularly  directed  to  the  fact,  that  the  distant  pil- 
lars of  the  rows  are  vastly  smaller  upon  the  canvas  than  the 
near  ones;  but  in  all  such  cases  the  mind  merely  uses  the  signs 
to  help  it  to  conceive  of  the  things  according  to  other  princi- 
ples of  judging;  so  in  any  common  case  of  seeing,  the  mind 
takes  so  little  account  of  the  apparent  size  of  objects  passing 
instantly  from  the  types  to  the  realities,  generally  known,  that 
it  soon  ceases  to  be  aware  that  the  apparent  size  of  the  same 
object  ever  changes.  Most  persons  would  be  surprised  to  be 
told,  £br  instance,  that  a  man  with  whom  one  is  shaking  hands, 
appears  to  the  mere  eye  ten  times  taller  than  when  he  has 
walked  ten  paces  away,  or  that  a  chair  at  one  end  of  a  room 
appears  to  a  person  sitting  at  the  other,  only  half  as  large  as  a 
chair  in  the  middle  of  the  room;  but  such  are  the  facts;  and 
they  may  be  immediately  proved  by  holding  a  common  eye- 
glass or  ring  at  a  certain  distance  from  the  eye,  and  then  look- 
ing through  it  at  any  similar  objects  placed  at  different  dis- 
tances; then,  while  of  a  chair  standing  near,  only  a  small  part 
will  be  visible  through  the  ring — of  a  distant  chair,  the  whole 
may  be  seen; — and  so  of  any  other  case.  At  five  miles'  dis- 
tance, Nelson's  fleet  on  the  great  day  of  Trafalgar  might  have 
been  seen  through  a  marriage-ring  as  the  picture  frame.  There 
are  occasions,  however,  where  the  usual  collateral  helps  to  the 
immediate  recognition  of  objects  being  wanting,  the  attention 
is  strongly  aroused  to  the  fact  of  their  diminutive  appearance 


176  LIGHT. 

produced  by  distance — for  instance,  when  a  man  first  from  the 
high  sea  approaches  a  land,  of  which  the  features  are  in  a  de- 
gree new  to  him:  an  Englishman  arriving  in  India  has  consi- 
derable difficulty  in  believing  the  little  specks  which  he  sees 
scattered  along  the  shore  to  be  commodious  dwellings,  and 
what  seem  to  him  only  luxuriant  herbs  or  bushes,  to  be  mag- 
nificent palm  trees. 

For  the  same  reason  that  a  distant  body  to  the  mere  eye  ap- 
pears diminutive,  viz.  the  smallness  of  the  visual  angle  be- 
tween the  extreme  points  observed,  so  does  a  distant  motion  to 
the  mere  eye  appear  slow.  A  carriage  dashing  past  a  pedes- 
trian in  the  street,  may  surprise  him  by  its  speed;  but  if  viewed 
at  the  same  time  by  a  spectator  from  the  top  of  St.  Paul's,  it 
seems  to  be  but  crawling  along  the  pavement.  A  ship  driven 
before  a  tempest,  scarcely  allows  the  sailor  on  board  to  distin- 
guish the  individual  masses  of  the  white  foam  through  which 
she  flies;  but  if  then  seen  on  the  distant  horizon  by  a  specta- 
tor on  shore,  she  is  scarcely  perceived  to  change  her  place.  A 
balloon  high  in  the  air,  and  borne  along  on  the  wings  of  the 
wind  at  the  rate  of  seventy  or  eighty  miles  an  hour,  may  still 
for  a  time  leave  a  spectator  on  earth  doubtful  as  to  the  direc- 
tion in  which  it  is  moving.  The  moon  in  her  orbit  wheels 
round  the  earth  at  the  astonishing  rate  of  about  2,000  miles  an 
hour,  yet,  owing  to  her  distance  from  it,  her  motion  is  not 
there  visible  to  the  naked  eye,  except  by  comparing  her  place 
at  considerable  intervals.  In  respect  to  bodies  still  more  dis- 
tant than  the  moon,  the  truth  at  present  under  consideration  is 
still  more  striking. 

Having  now  explained  how  the  apparent  transverse  measure  of 
bodies  and  of  space,  in  other  words  the  visual  angle  subtend- 
ed by  them,  is  affected  by  their  distance  from  the  eye,  we 
proceed  to  show  how  it  is  affected  also  by  their  position. 

A  globe  at  a  certain  distance  from  the  eye,  however  turned, 
has  the  same  appearance  or  bulk,  in  the  field  of  view,  and  its 
outline  traced  upon  glass  held  between  it  and  the  eye,  is  always 
a  circle;  but  an  egg,  although  when  held  in  one  position  it 
produces  a  circular  outline  or  image,  when  held  in  another, 


THE  EYE — FORE-SHORTENING.  177 

produces  an  image  nearly  oval.  A  wheel  when  viewed  side- 
ways appears  a  perfect  circle,  when  viewed  edgeways  it  ap- 
pears a  broad  straight  band  or  line,  and  when  in  any  interme- 
diate position  it  also  appears  oval.  The  apparent  form  then  is 
only  a  hint  to  the  mind  from  which,  by  former  experience  or 
other  means,  it  guesses  at  the  true  form.  If  a  man  had  never 
seen  an  egg  but  endways,  he  never  could  have  known  that  it 
was  not  a  perfect  sphere. 

If  any  long  straight  object,  as  a  beam,  be  placed  with  one 
of  its  ends  directly  to  the  eye,  that  end  only  can  be  seen,  and 
according  to  the  case  must  appear  a  square  or  circle  of  the  di- 
ameter of  the  beam;  if  it  then  be  placed  with  its  side  directly 
to  the  eye,  its  whole  length  will  be  seen;  and  if  placed  in  any 
intermediate  position,  it  will  appear  more  or  less  shortened; — 
in  all  cases,  its  outline  on  the  retina  being  similar  to  that  of  its 
shadow  on  a  wall  behind  the  person.  A  man  has  advanced  on 
a  spear  pointed  directly  to  his  eye  without  seeing  it,  or  on  a 
bar  of  iron  carried  on  the  shoulder  of  a  porter  met  in  the  street. 
A  common  telescope  held  with  its  end  to  the  eye  appears  a 
perfect  circle;  if  then  inclined  a  little,  it  seems  to  jut  out  on  one 
side,  and  as  the  inclination  is  increased,  it  juts  out  more  and 
more,  until  it  displays  its  whole  length.  A  great  ship  of  war 
whose  stern  is  towards  a  spectator,  appears  a  rounded  building, 
with  its  rows  of  windows  like  those  of  a  peaceful  habitation; 
but  as  it  turns,  it  gradually  reveals  the  bristling  cannon  in  their 
whole  length  of  fearful  batteries.  A  straight  row  of  a  thou- 
sand similar  objects,  as  of  soldiers  in  rank,  pillars,  trees,  &e., 
may  appear  to  a  person  at  the  extremity  as  only  one  object  of 
the  kind,  the  nearest  individual  completely  hiding  all  the  others; 
but  if  viewed  from  the  side  and  at  a  certain  distance,  the  indi- 
viduals may  be  counted. 

The  appearance  now  treated  of  is  called  fore-shortening, 
and  is  to  be  noted  wherever  any  surfaces  or  lines  are  not  placed 
so  as  completely  to  face  the  spectator. 

Perhaps  the  most  important  case  of  fore-shortening  which 
has  to  be  observed  is  that  of  an  extensive  plane  surface,  along 
which  the  eye  looks — for  instance,  the  general  surface  of  the 
earth  or  sea,  by  estimating  aright  the  fore-shortening  of  which, 

23 


178  LIGHT. 

we  partly  judge  of  the  distance  or  situation  of  the  objects 
placed  upon  it.  It  is  evident,  that  in  all  such  cases,  the  more 
distant  portions  of  the  surface  are  progressively  more  fore-short- 
ened than  the  nearer;  for  a  man  standing  on  a  plain  as  a  b,  and 
looking  down  immediately  before  him  with  his  eye  at  c,  sees  a 
portion  of  the  surface  almost  directly,  or  without  any  fore- 
shortening, and  an  extent  of  5  feet,  as  a  d  (if  5  feet  be  the 

height  of  the  eye,) 
will  subtend  in  the 
eye  an  angle  of  45°, 
viz.  the  angle  a  c  d, 
or  will  appear  45° 
long  in  his  field  of 

view — therefore  half  of  what  is  subtended  by  the  whole  space 
from  his  feet  to  the  horizon ;  the  next  five  feet  will  subtend  an 
angle  of  only  18°,  viz.  d  e  /,  the  next  of  S°,  viz.  f  c  g,  and 
so  on;  and  as  he  carries  his  view  more  and  more  forward,  the 
surface  becomes  to  it  more  and  more  oblique,  until  at  last  the 
light  coming  from  the  surface  seems  rather  to  skim  along  the 
level  than  to  rise.  This  explains  why  a  person  having  a  side 
view  of  a  row  of  separate  objects,  as  of  men  in  line,  trees, 
pillars,  &c.,  may  see  through  between  the  nearest  of  them,  but 
towards  the  extremes  of  the  view  sees  them  as  if  standing  in 
closest  possible  array,  or  as  if  forming  a  continued  surface. 
The  same  remark  explains  why  masses  of  cloud  scattered  uni- 
formly over  the  sky,  may  allow  a  spectator  to  see  wide  inter- 
vals of  the  blue  heaven  over  head,  while  all  around  there  is  a 
dense  cloudy  wall  appearing  to  rest  on  the  horizon. 

If  a  man  standing  on  a  hill  look  down  upon  a  field  or  plain 
which  is  well  known  to  him,  and  if  he  see  some  objects  near 
its  side,  and  some  near  its  middle,  and  some  near  its  distant 
border,  he  knows  at  once  how  far  they  are  from  him  and  from 
each  other.  Similarly,  if  viewing  the  ocean  from  a  lofty  cliff, 
and  seeing  ships  scattered  over  its  face,  he  may  judge  correctly 
of  their  distance,  for  he  can  see  only  a  certain  extent  of  ocean 
which  becomes  to  him  as  a  known  field.  The  man  stationed 
at  the  flag-staff  on  the  High  Knowl  peak  of  St.  Helena,  looks 
down  upon  a  circular  field  of  the  Atlantic  a  hundred  miles 


THE  EYE — PORE-SHORTENING.  179 

broad,  and  he  tells  the  distance  of  any  sail  in  sight  to  within  a 
mile  or  two.  Now  although  the  ground  plan  of  a  landscape 
may  not  be  so  level  as  the  field  or  ocean-face  now  spoken  of, 
there  is  still  an  approximation  to  the  true  plain,  which  very 
considerably  assists  a  spectator's  judgment  of  distances.' 

Painters  are  not  only  careful  to  fore-shorten  correctly,  ac- 
cording to  the  proportion  explained  above,  all  the  objects 
which  they  portray,  but  they  often  avail  themselves  of  the 
principle  to  produce  most  striking  effects.  For  instance,  Mar- 
tin, in  many  of  his  beautiful  designs,  by  judicious  fore-short- 
ening, has  exhibited  miles  in  extent  of  gorgeous  architecture 
and  of  armed  men,  on  a  space  of  canvas  that  would  seem 
scarcely  more  than  sufficient  to  receive  a  few  figures:  he  has 
made  a  single  magnificent  pillar  or  accoutred  warrior  placed  in 
the  foreground,  become  the  type  which  first  fills  the  mind  with 
admiration,  and  then  sends  it  along  the  retiring  lines  of  beau- 
tiful perspective,  where  every  tip  or  edge  renews  the  first  im- 
pression. A  man  lying  on  a  table  or  a  bed  nearly  as  high  as 
the  eye,  with  his  feet  towards  the  spectator,  is  fore-shortened 
into  a  roundish  heap,  of  which  the  soles  of  the  feet  hide  tho 
greater  part.  This  is  the  description  of  the  painting  which  has 
been  called  the  miraculous  entombment  of  Christ,  and  it  is  be- 
cause an  unreflecting  spectator  moving  sideways  with  the  ex- 
pectation of  seeing  more  of  the  body,  still  sees  only  the  soles 
of  the  feet,  and  may  suppose  the  body  turned  round  so  as  to 
front  him,  that  the  painting  has  received  its  appellation.  For 
nearly  the  same  reason  the  eyes  of  a  common  portrait  may 
seem  to  follow  a  spectator  to  whatever  part  of  the  room  he  goes. 
A  rifleman  represented  as  taking  aim  directly  in  front  of  the 
picture,  will  seem  to  have  in  his  power  every  spectator  stand- 
ing in  the  room;  for,  as  in  the  case  of  the  miraculous  entomb- 
ment, every  spectator  present  will  feel  as  if  he  alone  could  see 
the  picture  as  all  see  it.  To  terrify  young  ladies,  a  little  arch 
Cupid  has  ingeniously  been  represented  with  his  arrow  pointed 
directly  at  them,  and  just  ready  to  let  it  slip  from  his  bended 
bow: — and,  oh,  how  they  are  terrified  ! 

As  the  painter,  availing  himself  of  a  knowledge  of  the  prin- 
ciples now  explained,  by  which  the  eye  usually  judges  of  size 


180  LIGHT. 

and  distance,  may  produce  on  his  canvas  the  most  charming  il- 
lusions, so  may  the  tasteful  landlord  in  his  ornamental  gardens 
and  pleasure  grounds,  by  working  his  levels  into  artificial  un- 
dulation of  hill  and  dale,  with  magnitude  of  tree  and  of  edifice 
to  correspond — make  the  eye  of  a  spectator  luxuriate  in  the 
contemplation  of  supposed  extensive  plains,  lofty  mountains, 
distant  pagodas,  and  wide-spread  lakes — all  within  the  narrow 
space  of  an  acre  or  two; — thus,  in  truth,  by  other  means,  pro- 
ducing on  the  retina  the  same  impressions  as  Claude,  Poussin, 
or  Wilson,  by  their  finest  pictures. 

When  any  object  or  mass  of  objects  is  fore-shortened,  by 
one  part  being  farther  from  the  eye  than  another,  that  part  ap- 
pears also  in  a  proportion  smaller  than  the  other.  For  exam- 
ple, in  a  straight  row  of  similar  houses,  trees,  &c.,  those  near- 
est to  the  eye  will,  on  a  glass  held  before  the  eye  to  receive 
their  images,  form  the  largest  images,  and  there  will  be  a  gra- 
dual diminution,  from  the  largest  to  the  least,  so  that  lines 
drawn  upon  the  glass  along  the  tops  and  bottoms  of  the  images 
would  tend  to  a  point,  called,  for  a  reason  explained  below,  the 
vanishing  point.  Thus  a  person  looking  from  a  window  upon 
a  long  straight  street,  must,  to  see  to  the  chimneys  of  the  near- 
est house,  look  through  the  top  of  the  window,  and  to  see  the 
street-door  must  look  through  the  bottom;  but  the  most  dis- 
tant house,  both  top  and  bottom,  may  be  concealed  from  view 
by  a  little  spot  upon  the  glass  at  the  height  of  the  eye.  This 
remarkable  tapering  of  fore-shortened  objects  may  of  course 
be  strikingly  observed  on  looking  at  any  correctly  made  draw- 
ing or  engraving  meant  to  represent  a  retiring  row  of  similar 
objects; — such  drawing  being  in  truth  an  attempt  to  realize  by 
art  the  appearance  of  the  objects  as  seen  through  the  window. 

The  art  which  attempts  to  trace  objects  on  a  plane  surface,  as 
they  would  appear  on  looking  at  them  through  that  surface  if  it 
were  transparent,  with  their  various  degrees  of  apparent  dimi- 
nution on  account  of  distance,  and  of  fore-shortening  on  account 
of  obliquity  of  position,  is  called  from  the  Latin  word  signify- 
ing to  look  through,  the  art  of  perspective.  It  consists  en- 
tirely of  the  two  parts  now  mentioned;  and  notwithstanding 
the  terror  with  which  the  study  of  it  is  clothed  in  the  imagina- 


THE  EYE — PERSPECTIVE. 


181 


tions  of  many  young  painters,  by  reason  of  the  mathematical 
difficulties  with  which  it  has  usually  been  mixed  up,  it  is  in  it- 
self exceedingly  simple.  We  hope  that  a  person  capable  of  or- 
dinary attention  will,  from  what  we  have  already  said,  and  from 
the  few  additional  remarks  which  we  have  still  to  make  on  the 
appearances  of  nature,  be  able  completely  to  understand  the 
great  laws  of  perspective.  Although,  without  a  knowledge  of 
these  laws,  a  quick  eye  soon  enables  its  possessor  to  sketch  from 
nature  with  much  truth;  and  although  the  two  instruments,  the 
camera  obscura  and  camera  lucida,  give  almost  mathematical 
accuracy  to  drawings,  without  requiring  other  skill  in  the  drafts- 
man than  to  trace  with  ink  or  pencil  the  lines  which  he  sees  as 
if  on  the  paper,  still  the  subject  is  so  interesting  to  all  who  look 
either  at  nature  or  the  works  of  art,  that  no  intelligent  person 
should  neglect  it. 

Supposing  a  straight  row  of  similar  objects,  as  of  the  stone 
blocks  or  pillars  represented  here  from  a  to  S,  to  be  viewed  by 
a  person  standing  near  C,  then,  because,  as  already  explained, 

objects  to  the  eye  ap- 
pear smaller  in  exact 
proportion  to  their  in- 
creased distance  from 
it,  the  second  block,  if 
twice  as  far  off  as  the 
first,  would  appear  only 
half  as  large;  the  third, 
if  three  times  as  far, 
would  be  only  one- 
third  as  large,  and  so 
on  to  any  extent,  and  for  any  other  proportions;  and  if  the 
1,000th  or  any  other  nearer  or  more  distant  pillar  subtended  to 
the  eye  an  angle  less  than  the  sixtieth  of  a  degree  of  the  field 
of  view,  it  would  be  altogether  invisible,  even  if  nothing  inter- 
vened between  it  and  the  eye.  Then,  where  the  row  ceased 
to  be  visible  from  the  minuteness  of  the  parts,  or  from  the  fact 
of  the  nearer  objects  concealing  the  more  remote,  it  might  be 
said  to  have  reached  its  vanishing  point. 

Now  it  is  very  remarkable  that  in  any  such  case  of  a  straight 


183  LIGHT. 

line  or  row  vanishing  from  sight,  in  whatever  direction  it  points, 
east  for  instance,  although  the  eye  to  see  the  near  end  of  it 
would  have  to  look  about  north-east,  still  the  point  in  the  hea- 
vens, or  in  a  picture,  or  transparent  plane  before  the  eye,  where 
the  line  would  vanish,  would  be  exactly  east  from  the  eye,  and 
not  in  the  slightest  degree  either  to  the  north  or  to  the  south  of 
the  east  point,  because  the  pillars  happened  to  be  north  or  south 
of  the  individual;  and  therefore,  if  there  were  two  or  more 
rows  of  pillars  parallel  to  the  first,  but  considerably  apart  from 
each  other,  as  the  lines  here,  a  S,  b  S,  d  S,  &c.,  still  all  would 
vanish  or  seem  to  terminate  in  the  very  same  point  of  the  field 
of  view.  The  reason  of  this  is  easily  understood.  Let  us 
suppose  a  line  drawn  directly  east  from  the  eye  or  to  the  point 
tf,  viz.  a  line  directly  over  the  line  C  S,  and  that  the  line  of 
pillars  a  S,  also  pointing  east,  is  20  feet  north  of  the  spectator, 
and  the  line  of  pillars  b  S,  running  in  the  same  direction,  is  20 
feet  south  of  him,  then,  evidently,  for  the  same  reason  as  the 
space  between  the  top  and  bottom  of  the  pillars,  that  is  to  say 
their  height,  becomes  apparently  less  as  their  distance  from  the 
eye  increases,  so  will  the  space  between  each  pillar  and  the  point 
corresponding  to  its  place  in  the  visual  ray,  or  the  line  along 
which  the  eye  looks,  become  less,  and  the  lines  of  pillars  really 
20  feet  apart  from  the  visual  ray,  will,  at  a  certain  distance  from 
the  eye,  viz.  where  20  feet  is  apparently  reduced  to  a  point, 
appear  to  join  it,  and  the  three  lines  will  appear  to  meet  in  that 
point,  beyond  which  they  cannot  be  visible,  and  which  is  there- 
fore called  the  vanishing  point.  The  conception  of  this  truth 
may  be  facilitated  by  our  supposing  a  star  or  planet  to  be  rising 
in  the  eastern  point  of  the  heavens  at  the  moment  of  observa- 
tion; then,  if  the  three  parallel  lines  were  continued  on  to  the 
planet,  and  were  visible  as  far,  they  would  arrive  there  with 
the  20  feet  of  interval  between  them  just  as  they  left  the  earth; 
but  as  any  planet,  although  many  thousand  miles  in  diameter, 
owing  to  its  distance  from  the  earth,  appears  only  a  point,  much 
more  would  two  lines  only  20  feet  apart  be  there  undistin- 
guishable  in  place  by  human  sight.  And  what  is  true  of  a 
space  of  20  feet  between  parallel  lines,  is  equally  true,  as  re- 
gards human  vision,  of  a  space  of  hundreds  or  of  thousands  of 


THE  EYE — PERSPECTIVE. 


183 


miles:  as  a  general  rule,  therefore,  it  holds,  that  all  lines  in  reali- 
ty parallel  to  each  other  in  perspective  tend  to  and  finish  in  the 
same  vanishing  point,  viz.  the  situation  of  the  line  in  which  the 
eye  looks  when  directed  parallel  to  any  one  of  those  real  lines. 
And  this  is  true  not  only  of  lines  in  the  same  level  or  horizon- 
tal plane,  viz.  such  as  might  be  along  the  surface  of  the  sea, 
but  also  of  lines  that  are  vertical  or  one  above  another,  as  those 
running  along  the  tops  and  bottoms  of  the  pillars  here,  or  along 
the  roofs  and  windows  of  the  houses,  and  indeed  of  all  lines  in 
whatever  situation,  provided  they  are  parallel  to  the  visual  ray. 
When  it  is  ascertained,  therefore,  that  a  line  in  any  natural  or 
artificial  object  points  10  or  20  or  any  number  of  degrees  north 
or  south,  or  above  or  below,  &c.  the  centre  of  a  scene  or  pic- 
ture, that  is  to  say,  the  point  of  sight  or  principal  visual  ray, 
then  also  is  it  known  that  all  the  parallels  to  that  line  have  their 
vanishing  point  in  that  spot  of  the  field  of  view,  and  a  line  sup- 
posed drawn  from  the  eye  to  the  heavens,  or  really  drawn  to 
the  picture  in  that  direction,  marks  the  true  vanishing  point. 

It  is  explained  now,  why  in  a  long  arched  tunnel,  or  a  cathe- 
dral with  many  longitudinal  lines  on  its  floor,  walls,  roof,  &c., 
all  such  lines  seem  by  an  eye  looking  along  from  one  end,  ap- 
pear to  converge  to  a  point  at  the  other,  like  the  radii  of  a  spi- 
der's web;  and  why  in  the  representation  of  a  common  room, 
viewed  from  one  end,  all  the  lines  of  the  corners,  tops  and 

bottoms  of  windows, 
floor,  stripes  on  a  car- 
pet, corners  of  tables, 
&c.  being  parallel  to 
each  other,  tend  to  the 
same  vanishing  point, 
as  V,  and  are  cut  off 
according  to  the  rule 
of  fore-shortening  for- 
merly alluded  to.  The 
same  considerations 
will  explain  the  appearance  often  to  be  observed,  of  two  little 
clouds  near  each  other  and  almost  motionless  for  a  long  time 
in  the  distant  sky  directly  to  windward,  but  which  on  approach- 


184  LIGHT. 

ing  the  spectator,  appear  to  be  gradually,  and  at  last  suddenly 
enlarged,  while  one  of  them  sweeps  past  considerably  to  the 
right  hand,  and  the  other  considerably  to  the  left,  but  both  again 
meet,  when  at  the  former  distance  beyond  the  spectator,  ap- 
pearing there  as  small  as  at  first.  Clouds  being  so  mutable  and 
uncertain  in  their  forms,  persons  have  been  led  to  deem  all 
apparent  changes  in  them,  of  form,  size,  and  place,  to  be  real 
changes,  and  not,  as  they  generally  are,  mere  optical  or  per- 
spective illusion. 

By  far  the  most  important  vanishing  point  in  common  scenes, 
is  the  middle  of  the  horizon  or  level  line,  and  in  a  picture, 
properly  placed,  it  is  at  the  exact  height  of  the  eye.  It  is 
marked  S  in  the  figure  before  the  last,  and  V  in  the  last  figure. 
Because  in  houses,  the  roofs,  foundations,  floors,  windows,  &c. 
are  all  horizontal,  the  vanishing  points  of  their  lines  must  be 
somewhere  in  the  horizon,  and  if  the  spectator  be  in  the  mid- 
dle of  a  street  or  of  a  building,  and  be  looking  in  the  direction 
of  its  walls,  their  vanishing  point  will  be  in  the  centre  of  the 
scene  or  picture;  if  he  be  elsewhere,  it  will  be  at  one  side.  In 
holding  up  a  picture  frame,  through  which  to  view  a  scene  suit- 
able for  a  picture,  it  would  be  found  most  befitting  to  raise  it 
until  the  line  of  the  horizon  appeared  to  cross  at  about  one- 
third  from  the  bottom: — this  fact  becomes  the  reason  of  the 
rule  in  painting,  so  to  place  the  horizontal  line.  In  beginning 
a  picture,  this  line  is  usually  the  first  line  drawn  on  the  canvas, 
as  marking  the  place  of  the  vanishing  points  of  all  level  lines 
and  surfaces.  And  the  eye  of  the  spectator  is  supposed  to  be 
placed  before  the  middle  of  it,  and  generally  about  as  far  from 
the  picture  as  the  picture  is  itself  long,  such  being  the  extent 
of  view  which  the  eye  at  one  time  most  conveniently  com- 
mands. 

Understanding  now  that  the  apparent  or  perspective  direc- 
tion of  all  lines  in  a  scene  is  towards  their  vanishing  points  as 
above  discovered,  or  parallel  to  the  picture  where  the  originals 
are  so,  and  therefore  cannot  have  vanishing  points  according 
to  the  rule  given,  we  proceed  to  show  how  much  of  a  line 
drawn  to  any  vanishing  point  belongs  to  the  known  magnitude 
of  any  object  through  which  it  passes;  in  other  words,  how 


THE  EYE PERSPECTIVE. 


185 


much  an  object  is  in  perspective  fore-shortened  in  consequence 
of  its  obliquity  of  position. 

If  we  suppose  A  S  P  to  represent  a  plate  of  glass  seen  edge- 
ways, and  that  towards  the  point,  S,  in  it,  an  eye  is  looking 

from  the  point  D9 
evidently  then,  a 
line  from  P,  con- 
tinued in  the  di- 
rection P  R  until 
,  it  vanished  from 
sight,  could  have 
as  its  perspective  image  or  representation  on  the  glass  only  a 
line  reaching  from  P  to  S,  the  point  of  sight  here,  and  the 
pictorial  vanishing  point  of  the  line.  Now,  to  divide  the  repre- 
sentative line  P  S  so  as  to  correspond  with  any  given  portions 
of  the  original  line  P  R,  &c.,  it  would  only  be  necessary  to 
draw  other  lines  from  the  place  of  the  eye  D  to  cut  or  touch 
the  original  line  in  the  situations  desired,  and  these  lines  would 
cut  the  perspective  line,  S  P,  as  required:  for  instance,  the  por- 
tion of  the  true  line,  a  b,  would  be  represented  by  the  por- 
tion of  the  image  line,  S  P,  included  between  the  two  lines 
a  D  and  b  D,  and  so  of  any  other  portions.  There  are  figures 
drawn  on  many  mathematical  scales  by  which  such  problems 
as  this  can  be  solved  at  once;  and  the  proportions  are  also  de- 
tailed in  common  tables:  but  the  most  generally  convenient 
mode  in  practice  is,  to  set  off  on  the  intended  drawing,  (as 
that  of  which  c  d  here  marks  the  boundary,)  from  the  point 
of  sight,  S,  a  distance  on  the  horizontal  line,  at  D,  equal  to 


the  distance  of  the  eye  (rom  the  picture,  and  then  by  oblique 
lines  drawn  upon  the  base  line,  P  R,  to  cut  the  perpendicular 


LIGHT. 

line,  P  S,  in  the  situations  desired — as  is  seen  in  the  last 
sketch,  which  differs  from  the  present  only  in  having  the  point 
of  distance  marked  before  its  point  of  sight,  instead  of  late- 
rally, as  here.  And  the  line  P  S  being  always  cut  by  the  ob- 
lique line  from  D,  in  proportion  to  the  length  of  base  line  be- 
tween P  and  the  extremity  of  the  oblique  line,  a  horizontal  line, 
drawn  through  any  point  in  it,  cuts  in  corresponding  propor- 
tions all  the  other  lines  which  have  their  vanishing  points  in 
the  horizontal  line  S  D,  for  instance,  a  S,  b  S,  &c.  Thus,  to 
draw  in  perspective,  on  the  surface  above  represented  and  pre- 
pared, a  chess  board,  or  board  of  squares,  it  is  necessary  to 
set  off  the  breadth  of  the  board  on  the  base  line  to  the  right 
and  left  of  P,  viz.  to  b  and  a,  and  then  to  draw  to  the  point 
of  sight  as  a  vanishing  point,  the  lines  a  S  and  b  S,  part  of 
which  lines  will  therefore  represent  the  sides  of  the  board, 
and  then  to  draw  the  diagonal,  b  D,  which,  for  the  reasons 
above  stated,  will  cut  the  lines  P  S  and  a  S  in  proportion  to 
the  length  of  base  line  to  the  right  of  their  extremities;  a  efb, 
therefore,  is  a  square  seen  in  perspective,  and  any  number  of 
smaller  included  squares  are  made  by  drawing  lines  from  the 
vanishing  point  to  equal  divisions  on  the  base,  and  making 
cross  lines  where  the  diagonal  cuts  these. 

Much  of  the  delight  which  the  art  of  painting  is  calculated 
to  afford  is  lost  to  the  world,  because  persons  in  general  know 
not  how  to  look  at  a  picture.  Unless  a  spectator  place  himself 
where  he  can  see  the  objects  in  true  perspective,  so  that  he 
may  fancy  himself  looking  at  them  through  a  window  or  open- 
ing, every  thing  must  appear  to  him  falsely  and  distorted. 
The  eye  should  be  opposite  the  point  of  sight  of  the  picture, 
and  therefore  on  a  level  with  the  line  of  the  horizon,  and  it 
should  be  at  the  required  distance,  which  is  generally,  at  least, 
as  great  as  the  length  of  the  picture.  But  blame  not.  unfre- 
quently  rests  also  with  the  artist,  from  his  having  neglected 
the  study  of  perspective.  It  is  very  common,  for  instance,  to 
see  miniature  resemblances  of  architectural  structures  so  fore- 
shortened and  tapered,  that  the  eye,  to  see  them  in  true  per- 
spective, would  require  to  be  within  an  inch  of  the  paper; 
whence,  at  the  usual  distance  of  ten  or  twelve  inches,  they  are 


THE  BYE — JUDGING  OP  SIZE.  187 

seen  as  hideous  distortions.  The  specimens  in  the  few  pre- 
ceding pages  necessarily  exemplify,  in  a  degree,  this  error,  be- 
cause the  point  of  distance  had  to  be  marked  where  there 
was  but  a  small  page.  These  figures,  therefore,  by  any  per- 
son studying  the  subject  particularly,  should  be  drawn  on  such 
a  scale  as  that  the  eye  may  really  view  them  at  the  distance 
supposed. 

A  means  of  judging  of  the  dimensions  of  bodies  by  the  visual 
angle,  but  which  depends  neither  on  the  absolute  size  of  the 
image,  nor  on  the  fore-shortening  of  the  ground  plane  on 
which  the  body  stands,  is  to  use  known  objects  in  view  as 
measures  for  others  near  them  which  are  unknown. 

If  any  person  of  our  acquaintance  be  standing  at  some  dis- 
tance from  us  near  another  person  who  is  a  stranger,  we  know 
how  tall  the  stranger  is  by  taking  the  acquaintance  as  a  mea- 
sure. 

In  pictorial  representations  of  objects  little  familiar,  as  to 
many  people  are  the  Egyptian  pyramids,  the  bodies  of  the 
whale,  the  elephant,  the  camel,  &c.,  human  beings  may  be  re- 
presented around  them  to  serve  as  measures  for  the  less  known 
object.  The  Colossus  of  Rhodes,  seen  from  afar,  might,  to  a 
stranger,  have  appeared  but  an  ordinary  statue  of  a  man;  but 
the  exact  magnitude  would  have  been  known  as  soon  as  a  ship 
of  known  dimensions  were  seen  sailing  into  port  between  his 
gigantic  limbs. 

When  an  unpractised  eye  is  first  directed  to  a  great  ship  of 
war,  it  will  on  many  accounts  dwell  upon  it  with  wonder  and 
admiration;  but  it  may  not  judge  truly  of  the  enormous  mag- 
nitude until  near  enough  to  perceive  the  sailors  climbing  on  the 
rigging,  and  appearing  there,  by  comparison,  as  flies  or  little 
birds  appear  among  the  branches  of  a  majestic  tree. 

By  having  a  measure  of  this  kind  presented  to  us,  the  mag- 
nitude and  elevation  of  some  fine  edifices  are  rendered  more 
obvious.  The  magnificent  pile  of  St.  Paul's  in  London  be- 
comes more  striking  still,  when  we  discover  visiters  looking 
from  the  balconies  near  the  summit  cross.  They  appear  so 
minute  among  the  surrounding  huge  masses  that  a  person  is  at 


I8<  LIGHT. 

iirst  for  a  while  disposed  to  doubt  whether  they  be  men;  but 
the  fact  once  ascertained,  the  grandeur  of  the  temple  is  render- 
ed extremely  impressive. 

Many  persons  cansfot  distinguish  between  the  little  pilot  bal- 
loon (sometimes  despatched  before  a  great  one  to  show  the  di- 
rection of  the  wind)  and  the  great  balloon  itself,  until  with  the 
last  they  perceive  the  aeronauts  as  little  black  points  suspended 
under  the  globular  cloud. 

Strangers,  visiting  Switzerland,  on  first  entering  the  valleys 
there,  are  often  much  deceived  as  to  their  extent.  Because 
familiar  generally  with  more  lowly  hills  and  shorter  valleys  at 
home,  but  which  from  being  near  to  the  eyes  form  bulky 
images,  and  having  no  measure  at  first,  they  almost  universal- 
ly underrate  the  Alpine  dimensions: — they  will  wonder,  for 
instance,  in  the  valley  of  Chamouny,  that  they  should  be  tra- 
Telling  swiftly  for  hours  without  reaching  the  end,  where  on 
entering  they  did  not  believe  the  length  to  be  as  many  miles. 

The  author  once  sailed  through  the  Canary  Islands,  and 
passed  in  view  of  the  far-famed  Peak  of  Teneriffe.  It  had 
been  in  sight  in  the  afternoon  of  the  preceding  day,  at  a  dis- 
tance of  more  than  100  miles,  appearing  then  only  as  an  ordi- 
nary distant  hill  rising  out  of  the  ocean;  but,  next  morning, 
when  the  ship  had  arrived  within  about  twenty  miles  of  it, 
and  while  another  ship  of  the  fleet,  holding  her  course  six  miles 
nearer  to  the  land,  served  as  a  measure,  it  stood  displayed  as 
perhaps  the  most  stupendous  single  object  which  on  earth,  and 
at  one  view,  human  vision  can  command.  That  noble  ship, 
whose  side  showing  its  tiers  of  cannon,  equalled  in  extent  the 
fronts  of  ten  large  houses  in  a  street,  and  whose  masts  shot  up 
like  lofty  steeples,  appeared  as  a  speck  scarcely  rising  from  the 
sea,  compared  with  the  huge  prominence  beyond  it  towering 
sublimely  to  heaven,  and  around  which  the  masses  of  cloud, 
although  as  lofty  as  those  which  sail  over  the  fields  of  Britain, 
were  still  hanging  low  on  its  sides.  Teneriffe,  alone,  of  very 
high  mountains,  rises  out  of  the  bosom  of  the  ocean  with  in- 
accessible steepness  on  one  side,  to  an  elevation  of  13,000  feet; 
and  as  an  object  of  contemplation,  therefore,  is  more  impres- 
sive than  even  the  still  loftier  summits  of  Chimborazo  or  the 


THE  EYE — JTTDGING  OP  SIZE.  189 

Himalayas,  which  rise  from  elevated  plains,  and  in  the  midst 
of  surrounding  hills. 

It  is  because  objects  which  are  nearly  on  a  level  with  us,  as 
contrasted  with  such  as  are  either  much  above  or  much  be- 
low, are  in  general  more  numerously  surrounded  by  other 
objects  which  serve  as  measures  of  comparison,  that  we  judge 
BO  much  more  correctly  of  the  size  and  distance  of  the  for- 
mer than  of  the  others. 

A  man  walking  like  ourselves  on  the  sea-shore  or  other  le- 
vel, is  at  once  recognised;  and  probably  it  may  not  occur  to 
us,  that  he  appears  smaller  on  account  of  the  distance;  but  if 
the  same  man  be  seen  afterwards  at  an  equal  distance  above  us, 
collecting  the  sea  fowl's  eggs  on  the  face  of  a  cliff,  or  below 
us,  gathering  shells  on  the  beach,  when  we  ourselves  have 
reached  the  height,  he  appears  no  bigger  than  a  crow:  yet  in 
all  the  cases  he  is  where  the  same  bulk  forms  the  same  magni- 
tude of  image  on  the  retina. 

Even  on  a  horizontal  plain,  if  the  general  surface  be  bare  and 
uniform,  single  distant  objects  appear  very  diminutive.  This  is 
true,  for  instance,  of  a  man  seen  apart  from  his  caravan,  while 
journeying  across  a  sandy  desert;  while  a  man  viewed  at  an 
equal  distance,  in  the  midst  of  a  cultivated  landscape,  appears  of 
his  natural  size;  the  same  is  true  of  a  boat  or  ship  seen  out  on 
the  high  sea,  as  contrasted  with  similar  objects  viewed  in  a  har- 
bour, where  other  known  objects  are  near  them. 

We  may  now  understand  why  the  sun  and  moon,  when  rising 
or  setting,  appear  to  us  much  larger  than  when  they  have  at- 
tained meridian  height — although,  if  we  examine  them  by  any 
measure  of  the  visual  angle,  as  simply  by  looking  at  them  through 
the  same  ring  or  tube,  we  find  that  there  is  no  difference.  The 
Bun  and  moon  in  appearance  from  this  earth  are  nearly  of  the 
same  size,  viz.  always  occupying  in  the  field  of  view  about  the 
half  of  a  degree,  or  as  much  as  is  occupied  by  a  circle  of  a  foot 
in  diameter  when  held  about  250  feet  from  the  eye — which  cir- 
cle, therefore,  at  that  distance,  and  at  any  time,  would  just  hide 
either  of  them.  Now,  when  a  man  sees  the  rising  moon  appa- 
rently filling  up  the  end  of  a  street,  which  he  knows  to  be  100 


190  LIGHT. 

feet  wide,  he  very  naturally  believes  that  she  then  subtends  a 
greater  angle  than  usual,  until  the  reflection  occur  to  him,-— 
which  it  rarely  will  of  itself,  that  he  is  using  as  a  measure  of  her 
size,  a  street  known  indeed  to  be  100  feet  wide,  but  of  which  the 
part  concerned,  owing  to  its  distance,  appears  to  his  eye  exceed- 
ingly small.  The  width  of  the  street  near  him  may  occupy  60° 
of  his  field  of  view,  and  he  might  see  from  between  the  houses 
many  broad  constellations  instead  of  the  moon  only;  but  the 
width  of  the  street  far  off  may  not  occupy,  in  the  same  field  of 
view,  the  twentieth  part  of  a  degree,  and  the  moon,  which  al- 
ways occupies  half  a  degree,  will  there  appear  comparatively 
large.  The  kind  of  illusion  now  spoken  of  is  yet  more  remark- 
able when  the  moon  is  seen  rising  near  still  larger  known  ob- 
jects,— for  instance,  beyond  a  town  or  a  hill  which  then  appears 
within  her  luminous  circle.  Any  person  who  from  the  river- 
side terraces  of  Greenwich  has  observed  the  sun  setting  beyond 
London,  with  St.  Paul's  cathedral  included  in  the  glorious  pic- 
ture, will  recollect  a  most  interesting  example  of  our  present 
subject.  That  our  ocular  judgment  of  the  size  of  the  sun  or 
moon  is  thus  influenced  by  ihe  presence  or  absence  of  objects  of 
comparison,  and  not  by  the  place  of  the  bodies  in  the  sky,  is 
proved  by  the  fact  that  a  person  viewing  these  bodies  from  the 
bottom  of  some  of  the  Swiss  valleys,  where  he  might  almost 
suppose  himself  placed  at  the  centre  of  the  earth,  and  looking 
abroad  along  an  endless  extent  of  precipices — if  he  can  closely 
compare  them  with  certain  known  magnitudes  of  ridge  or  forest 
bounding  his  view,  sees  them,  although  at  a  great  elevation, 
as  large  as  they  appear  from  other  situations  when  rising  direct- 
ly out  of  the  sea.  Another  proof  is  afforded  by  the  case  of  a 
balloon  at  a  great  elevation  seen  crossing  the  disk  of  the  sun  or 
moon,  and  appearing,  however  large,  as  an  absolute  speck  with- 
in the  vast  luminous  area  In  a  future  paragraph  it  will  be  ex- 
plained, that  the  sun  and  moon  when  low  appear  larger  than 
when  high,  also,  because  of  their  apparent  dimness  when  low. 

It  may  be  remarked  here,  that  the  visual  estimate  formed  of 
the  great  size  of  the  sun  and  moon  when  seen  on  the  horizon,  is 
not  an  illusion,  as  is  popularly  supposed,  but  an  approximation 
to  truth,  still  vastly  short  of  the  reality.  When  we  see  a  tree, 


THE  EYE INTENSITY  OF  LIGHT.  191 

or  a  house,  or  a  bill,  apparently  within  the  circumference  of  one 
of  these  orbs,  it  is  really  true  that  the  orb  is  larger  than  the  tree, 
or  house,  or  hill,  and  so  much  larger,  that  even  if  the  whole 
British  Isles  could  be  lifted  away  from  the  earth,  and  suspended 
as  a  map  in  the  sky,  when  brought  near  the  moon  they  would 
hide  from  a  spectator  on  earth  but  a  small  part  of  her. 

Having  now  shown  that  in  relation  to  any  object,  the  visual  an- 
gle or  apparent  size  can  be  a  measure  of  the  true  size  only 
when  the  distance  also  is  known,  and  that  the  visual  angle  it- 
self serves  to  determine  the  distance  only  in  certain  cases,  we 
proceed  to  examine  other  means  which  the  eye  commands  for 
guessing  at  distances. 

2d.  Intensity  of  light,  shade,  and  colour.  (See  the  Analysis, 

page  122.) 

It  has  already  been  explained  that  light,  like  every  other  in- 
fluence radiating  from  a  centre,  becomes  rapidly  weaker  as  the 
distance  from  the  centre  increases,  being,  for  instance, only  one 
fourth  part  as  intense  at  double  distance,  and  in  a  corresponding 
proportion  for  other  distances;  while  it  is  still  farther  weakened 
by  the  obstacle  of  any  transparent  medium  through  which  it 
passes.  Now,  the  eye  soon  becomes  sufficiently  familiar  with 
these  truths  to  judge  from  them,  with  considerable  accuracy,  of 
the  comparative  distances  of  objects. 

The  fine  Gothic  pile  of  Westminster  Abbey  may  break  upon 
the  view  in  some  situation  where  nearer  edifices,  and  perhaps 
some  minor  imitations  of  its  beauties,  already  fill  and  dazzle  the 
eye  with  their  brightness;  but  the  mistf  or  less  d.stinct  outline 
of  the  former  warns  the  approaching  stranger  of  its  true  magni- 
tude, and  prepares  him  for  the  enjoyment  which  a  nearer  in- 
spection of  its  grandeur  and  perfection  is  to  afford. 

A  small  yacht  or  pleasure  boat  may  be  buili  from  the  same  mo- 
del or  of  the  same  comparative  dimensions  as  a  first-rate  vessel  of 
war,  and  may  be  in  view  from  the  shore  at  the  same  time,  only 
so  much  nearer  than  the  ship,  that  both  shall  form  images  of  the 
same  magnitude  on  the  retina  of  a  spectator.  In  such  a  case,  to 
an  unpractised  eye,  it  might  be  difficult  to  detect  the  difference; 


192  LIGHT, 

but  to  another,  the  bright  lights  of  the  little  vessel,  contrasted 
with  the  softer  or  more  misty  appearance  of  the  larger,  would 
leave  no  room  for  doubt.  A  haziness  occurring  in  the  atmos- 
phere between  the  little  vessel  and  the  eye,  might  considerably 
disturb  the  judgment. 

In  a  fleet  of  ships,  if  the  sun's  direct  rays  fall  upon  one  here  and 
there  through  openings  among  the  clouds,  while  the  others  re- 
main in  shade,  the  former  immediately  start  in  appearance  towards 
the  spectator.  Similarly,  the  mountains  of  an  unknown  coast, 
if  the  sunshine  fall  upon  them,  appear  comparatively  near;  but 
if  clouds  again  intervene,  they  recede  and  mock  the  awakened 
hope  of  the  approaching  mariner. 

A  conflagration  at  night,  however  distant,  appears  to  specta- 
tors generally,  as  if  very  near,  and  inexperienced  persons  often 
run  towards  it  with  hope  of  arriving  immediately,  but  find  after 
miles  travelled  that  they  have  made  but  a  little  part  of  their  way. 

A  person  ignorant  of  astronomy  deems  the  heavenly  bodies 
vastly  nearer  to  the  earth  than  they  are,  merely  because  of  their 
being  so  bright  or  luminous.  The  evening  star,  for  instance, 
Been  in  a  clear  sky  over  some  distant  hill-top,  appears  as  if  a 
dweller  on  the  hill  might  almost  reach  it — for  the  most  intense 
artificial  light  that  could  be  placed  on  the  height  would  be  dim 
in  comparison  with  the  beauteous  star;  yet  to  a  dweller  on  the 
hill  it  appears  just  as  distant  as  to  one  on  the  plain;  nay,  at 
thousands  of  miles  nearer,  the  appearance  would  still  be  nearly 
the  same. 

The  concave  of  the  starry  heavens  appears  flattened  above,  or 
nearer  to  the  earth  in  its  zenith  than  towards  its  horizon,  because 
the  light  from  above  having  to  pass  through  only  the  depth  or 
thickness  of  the  atmosphere,  is  little  obstructed,  while  of  that 
which  darts  towards  any  place  horizontally  through  hundreds  of 
miles  of  dense  vapour-loaded  air,  only  a  small  part  arrives. 

The  sun  and  moon  appear  larger  at  rising  and  setting  than, 
when  midway  in  heaven,  partly,  as  already  explained,  because 
while  below  they  can  easily  be  compared  with  other  objects,  of 
which  the  size  is  known,  but  partly,  also,  because  of  their  less 
light  in  the  former  situation,  while  their  diameters  are  always 
the  same. 


THE  EYE LIGHT  AND  SHADE.  193 

A  fog  or  mist  is  said  to  magnify  objects  seen  through  it.  The 
truth  is,  that  by  reason  of  the  diminished  intensity  of  light,  it 
makes  them  appear  farther  distant  without  lessening  the  visual 
angles  subtended  by  them;  and  because  an  object  at  two  miles, 
subtending  the  same  angle  as  an  object  at  one  mile,  must  be 
twice  as  large,  the  conclusion  is  drawn  that  the  dim  object  is 
large.  Thus  a  person  in  a  fog  may  believe  that  he  is  approach- 
ing a  great  tree,  fifty  yards  distant,  when  the  next  step  throws 
him  into  the  bush  which  had  deceived  him. — Two  friends  meet- 
ing in  a  fog  have  often  mutually  mistaken  each  other  for  persons 
of  much  greater  stature. — A  row  of  foxglove  flowers  on  a  neigh- 
bouring bank  has  been  mistaken  for  a  company  of  scarlet-clad 
soldiers  on  the  more  distant  face  of  the  hill.  There  are,  for  si- 
milar reasons,  frequent  misjudgings  in  late  twilight  and  early 
dawn. — The  purpose  and  effect  of  a  thin  gauze  screen  interposed 
between  the  spectators  in  a  theatre  and  some  person  or  object 
meant  to  appear  distant,  are  intelligible  on  the  same  principle:  a 
boy  near,  so  screened,  will  appear  to  be  a  man  at  a  distance. — 
The  art  of  the  painter  uses  sombre  colours  when  his  object  is  to 
produce  in  his  picture  the  effect  of  distance. — On  the  alarming 
occasion  of  a  very  dense  fog  coming  on  at  sea,  where  the  ships 
of  a  fleet  are  near  to  each  other,  without  wind,  and  where  there 
is  considerable  swell  or  rolling  of  the  sea,  much  damage  is  of- 
ten done;  but  it  is  to  be  remarked  in  such  a  case,  that  the  size  of 
ships  approaching  to  the  shock  is  always  in  idea  exaggerated. 

The  celebrated  Spectre  of  the  Brocken,  among  the  Hartz 
Mountains,  is  a  good  illustration  of  our  present  subject.  On  a 
certain  ridge,  just  at  sunrise,  a  gigantic  figure  of  a  man  had  of- 
ten been  observed  walking,  and  extraordinary  stories  were  re- 
lated of  it.  About  the  year  1800  a  French  philosopher  went 
with  a  friend  to  watch  the  phenomenon;  but  for  many  morn- 
ings they  had  paraded  on  an  opposite  ridge  in  vain.  At  last, 
however,  they  discovered  the  monster,  but  he  was  not  alone; 
he  had  a  companion,  and  singularly  he  and  his  companion  aped 
all  the  motions  and  attitudes  of  the  observer  and  his  companion: 
in  fact,  the  spectres  were  merely  shadows  of  the  observers, 
formed  by  the  horizontal  rays  of  the  rising  sun  falling  on  the 
morning  fog  which  hovered  over  the  valley  beyond;  but  be- 

25 


194  LIGHT. 

cause  the  shadows  were  very  faint,  they  were  deemed  distant, 
and  therefore  seemed  men  walking  on  the  opposite  ridge,  and 
because  a  comparatively  small  figure  seen  near,  but  supposed 
distant,  appears  of  gigantic  dimensions,  these  shadows  were  ac- 
counted giants. 

While  under  common  circumstances  the  comparative  inten- 
sities of  light  furnish  an  indication  of  difference  of  distance, 
there  are  other  cases  of  comparative  intensity,  in  which  bodies 
are  to  be  considered  not  as  wholes,  but  as  made  up  of  parts  un- 
equally exposed  to  the  source  of  light,  and  therefore  reflecting 
it  to  the  eye,  or  being  illuminated  in  different  degrees.  In  ob- 
serving, for  instance,  a  white  house  exposed  to  the  sun,  it  is 
seen  that  the  side  directly  receiving  the  rays  is  highly  illumi- 
nated or  bright,  while  the  other  sides  are  less  so,  and  are  said 
to  be  in  the  shade:  and  they  are  luminous  in  proportion  to  other 
sources  of  reflected  light  near  them.  The  different  faces  or 
walls  of  such  a  house  are  as  strongly  distinguished  from  each 
other,  by  the  mere  difference  of  shade,  as  if  they  were  of  dif- 
ferent colours.  If  the  object  were  a  ball  instead  of  a  square 
house,  there  would  still  be  as  great  differences  of  shade  in  the 
half  not  receiving  direct  rays,  but  the  parts,  instead  of  forming 
abrupt  contrasts  like  ther  walls  of  a  house,  would  melt  into  each 
other  and  mark  the  beautiful  round  contour  of  the  object.  The 
consideration  of  all  such  cases  forms  the  subject  of  chiaro-oscu- 
ro,  so  interesting  to  the  painter. 

Had  there  not  been  in  nature  the  provision  of  light  and  shade 
now  described,  the  sense  of  sight  would  have  been  of  compa- 
ratively little  use;  for  it  is  that  provision  chiefly  which  enables 
us  to  distinguish  the  profile  or  outlines  of  different  bodies  placed 
near  to  each  other,  and  the  protuberant  or  other  form  of  the 
surfaces  next  the  observer.  But  for  this,  it  would  have  been 
impossible  to  distinguish,  for  instance,  between  a  flat  circle,  a 
sphere,  and  a  cone,  all  directly  exposed  to  the  eye;  but,  in  re- 
ality, the  uniformly  bright  surface  of  the  circle,  the  soft  rounded 
shadowing  of  the  sphere,  and  the  shade  coming  to  a  point  on 
the  cone,  at  once  declare  the  true  forms.  But  for  the  shadows, 
the  fag.ade  of  a  white  palace  of  varied  architecture  would  have 
been  an  unmeaning  sheet  of  light;  the  lights,  however,  and 


THE  EYE — LIGHT  AND  SHADE.  195 

shadows  produced  by  the  juttings  and  recesses,  mark  the  varie- 
ty of  surface  most  completely:  and  the  round  pillar  is  distin- 
guished from  the  square,  and  every  pediment,  and  capital,  and 
architectural  ornament,  stands  out  pleasingly  conspicuous.  But 
for  light  and  shade,  again,  the  human  face  divine  would  have 
been  an  unmeaning  patch  of  flesh,  for  there  are  few  other  lines 
in  it  than  those  made  by  different  exposures  to  the  light,  and 
yet  every  prominence  and  depression  are  so  truly  indicated  to 
the  eye  that  it  becomes  full  of  meaning  or  expression.  How 
clearly  mere  light  and  shade  serve  to  convey  all  that  the  eye 
can  learn  of  a  scene  or  object,  may  be  perceived  by  examining 
any  of  the  admirable  engravings  which  now  abound — scarcely 
inferior  in  expression  to  the  most  finished  paintings. 

The  student  of  painting  soon  learns  that  the  lines  called  out- 
lines, by  which  he  first  sketches  subjects,  do  not  exist  at  all  in 
nature,  and  have  to  be  again  effaced  in  his  finished  work:  for 
they  only  mark  the  place  where  lights  and  shades  happen  to 
meet.  Much  may  be  conveyed  to  the  mind,  however,  by  a 
mere  outline,  and  particularly  if  lines  of  different  breadth  or 
thickness  are  used  to  mark  the  situation  of  the  lighter  and  deeper 
shadows. 

The  subject  of  chiaro-oscuro  is  not  so  simple  as,  from  the  fact 
of  the  sun  being  the  great  source  of  light,  might  at  first  be 
supposed;  for  although  this  be  true,  still  every  body  which  re- 
flects the  sun's  light  becomes  a  new  source  to  those  about  it, 
and  the  shading  of  a  picture  must  have  reference  to-  all  such 
sources,  and  to  the  colours  of  the  body  itself,  and  of  the  neigh- 
bouring bodies. 

In  looking  at  an  extended  landscape,  it  is  seen  that  the  near 
objects  considered  as  wholes,  are  comparatively  bright,  that 
their  shadows  are  strongly  marked,  and  that  their  peculiar  co- 
lours are  every  where  easily  distinguishable — as  of  flowers, 
fruit,  foliage,  &c.,  but  farther  off  the  colours  become  dim,  the 
lights  and  shadows  melt  into  each  other  or  are  confused,  and 
the  illumination  altogether  becomes  so  faint  that  the  eye  at  last 
sees  only  an  extent  of  distant  blue  mountain  or  plain — appear- 
ing bluish  because  the  transparent  air  through  which  the  light 
must  pass  has  a  blue  tinge,  and  because  the  quantity  of  light 


LIGHT. 

arriving  through  the  great  extent  of  air  is  insufficient  to  exhibit 
the  detail.  The  ridge  called  Blue  Mountains  in  Australia,  and 
another  of  the  same  name  in  America,  and  many  others  else- 
where, are  not  really  blue,  for  they  possess  all  the  diversity  of 
scenery  which  the  finest  climates  can  give;  but  to  the  discover- 
er's eye,  bent  on  them  from  a  distance,  they  all  at  first  appeared 
blue,  and  they  have  ever  since  retained  the  name. 

In  a  picture  by  an  artist,  who  on  his  canvas  stretched  on  a 
frame  disposes  the  lights,  shades,  and  colours  in  the  very  situ- 
ations and  with  the  intensities  which  on  coming  from  the  land- 
scape to  the  eyes,  through  a  plate  of  glass  filling  up  the  frame, 
they  would  have  had,  all  that  we  have  now  been  saying  is  strict- 
ly exemplified.  In  the  fore-ground  the  objects  are  large  and 
bright,  but  as  the  scene  is  supposed  to  be  gradually  more  re- 
mote, the  size  and  brightness  of  the  objects  correspondingly  di- 
minish, until  at  last  there  is  only  a  dim  mixture  of  bluish  or 
grayish  masses  forming  the  horizon  and  sky. 

A  child,  during  what  may  be  called  the  education  of  the 
sense  of  sight,  has  a  strong  perception  of  the  vast  differences 
of  appearance  which  things  assume  according  to  their  acciden- 
tal distance  from  the  eye,  their  position,  their  exposure  to 
light,  &c.;  for  these  differences,  being  often  calculated  to  de- 
ceive the  young  judgment,  many  of  them  have  been  noted 
with  surprise.  Thus,  a  boy  when  he  first  discovers  that  a  ship, 
which  at  the  quay,  with  sails  spread,  concealed  from  him  half 
the  heavens,  is  in  an  hour  or  two  afterwards  seen  by  him  on 
the  distant  horizon  as  a  speck  hardly  big  enough  to  hide  one 
star; — or,  again,  when  he  discovers  that  the  faint  blue  un- 
ehanging  mass  which  he  had  always  observed  bounding  in  one 
direction  the  view  from  the  home  of  his  infancy,  is  a  distant 
mountain-side  thickly  inhabited,  and  covered  with  fields  and 
gardens,  where  in  succession  all  the  bright  colours  of  the  dif- 
ferent seasons  predominate,  his  attention  is  strongly  awakened, 
and  he  feels  surprise.  But  as  soon  as  experience  has  enabled 
him  to  interpret  readily  and  correctly  the  visual  signs  under 
every  variety  of  circumstance,  his  attention  passes  so  instant- 
ly from  them  to  the  realities  which  alone  are  interesting  to 
him-— just  as  it  might  pass  from  the  paper  and  printing  of  ? 


THE  EYE — PERSPECTIVE.  197 

newspaper  to  the  important  intelligence  communicated  by 
them,  that  he  very  soon  ceases  to  be  aware  that  the  sign,  which 
in  every  case  similarly  suggests  the  object,  is  not  also  in  every 
case  similar  to  itself,  and  the  very  same  true  and  complete  re- 
presentation of  the  reality.  The  prejudice  that  the  sign  is  of 
this  nature,  becomes  quickly  so  strong,  that  even  a  difficult  ef- 
fort has  to  be  made  by  a  grown  person  again  to  attend  to  the 
mere  appearances,  in  any  scene  of  which  the  realities  are 
known. 

This  attempt  to  analyze  the  appearances,  and  which  in  one 
sense  is  a  trying  to  unlearn  something,  or  to  retrograde,  is 
called  the  study  of  perspective — and  when  it  regards  the  appa- 
rent reduction  of  size,  and  the  fore-shortening  of  bodies  under 
various  circumstances,  it  is  called  linear  perspective;  when  it 
regards  the  fading  of  light  and  the  modifying  of  colour,  it  is 
called  aerial  perspective.  As  the  art  of  painting  depends  en- 
tirely upon  the  understanding  of  these  two  departments,  the 
gradual  progress  which  it  has  made  in  different  countries  is  a 
measure  of  the  degree  in  which  the  common  prejudice  that 
things  appear  as  they  are  has  in  them  been  overcome.  Where 
this  prejudice  exists,  any  untaught  person  conceives  a  good 
painting  to  be  merely  a  miniature  representation  drawn  accord- 
ing to  a  certain  reduced  scale, — as  of  an  inch  to  a  yard, — and 
in  which  all  the  dimensions  of  things  are  to  be  measured  as 
simply  as  in  the  reality — while  the  colours,  as  to  vividness,  &c., 
should  perfectly  agree  in  both.  This  statement  is  remarkably 
illustrated  by  the  fact,  that  children  in  their  rude  attempts  to 
paint,  always  aim  at  realizing  the  notion  of  the  art  given  above, 
and  that  such  has  been  the  first  stage  of  painting  in  every  coun- 
try. In  Europe  now,  owing  to  the  labours  of  men  of  genius, 
art  in  painting  may  be  said  almost  to  rival  nature,  producing 
scenes  as  lovely  as  the  finest  of  nature's  scenes,  and  scarcely 
distinguishable  from  them,  but  in  other  countries,  as  in  China 
and  India,  among  the  native  artists,  the  first  stage  of  the  art  is 
still  in  existence.  In  a  Chinese  picture,  owing  to  the  absence 
of  perspective  proportions,  an  extensive  subject  is  only  a  col- 
lection of  portraits  of  men  and  things  drawn  on  the  same 
scale,  and  placed  near  one  another,  and  where  all  the  colour? 


19S  LIGHT. 

are  as  vividly  shown  as  if  the  objects  were  only  a  few  feet 
from  the  eye:  there  the  figures  at  the  bottom  or  fore-ground 
are  supposed  to  represent  the  objects  nearest  to  the  spectator, 
while  the  figures  higher  up  are  supposed  to  be  of  more  remote 
objects,  all  appearing  as  they  might  be  seen  in  succession  by  a 
person  who  had  the  power  of  flying  over  the  country.  This 
kind  of  picture  or  representation,  although  not  natural  if  all 
viewed  at  once,  may  communicate  more  information  than  a 
single  common  painting,  for  it  is  equivalent  to  many  such.  In 
Europe,  and  lately,  the  principle  has  been  usefully  acted  upon 
for  certain  purposes,  as  for  representing  on  one  long  sheet,  or 
on  a  succession  of  sheets  connected  in  a  suitable  manner,  the 
banks  of  a  river  or  a  line  of  road.  The  banks  of  the  Rhine, 
particularly,  have  thus  been  admirably  portrayed,  so  that  the 
spectator,  directing  his  eye  along  the  paper,  feels  almost  as  if 
carried  in  a  balloon  to  view,  in  great  detail,  the  whole  of  the 
real  and  enchanting  scenery.  The  principle  might  perhaps 
with  advantage  be  acted  upon  still  more  extensively — for  in- 
stance, to  produce,  instead  of  common  maps  or  charts  of  coun- 
tries, true  bird's-eye  views,  over  which  the  eye,  moving  from 
place  to  place,  at  every  new  point  of  sight,  would  see  a  cer- 
tain portion  of  the  country  just  as  a  bird  or  aeronaut  would, 
the  sketch  being  supposed  to  be  taken  from  that  certain  eleva- 
tion deemed  most  suitable  for  the  ends  in  view. 

3d.  Divergence  of  the  rays  of  light. 

This  is  the  next  circumstance  to  be  mentioned,  by  which  the 
eye  judges  of  distance.  Supposing  the  line  E  F  to  mark  the 
place  or  breadth  of  the  pupil  of  the  eye,  the  light  entering 

from  an  object  at «, which 
*s  near,  is  very  divergent, 
or  js  spreading  with  a 

J?  large  angle;   from  b  the 

extreme  rays  are  less  divergent,  or  they  open  at  a  smaller  an- 
gle; from  c  they  are  less  divergent  still,  and  so  on.  Now,  the 
eye,  to  form  an  image  on  its  retina,  requires  to  exert  a  bend- 
ing power  exactly  proportioned  to  the  divergence  jf  the  rays; 
and  it  appears  to  have  a  sense  of  the  effort  made,  which  be- 


THE  EYE DIVERGENCE  Ol'  KAYS.  199 

comes  to  the  person  a  kind  of  measure  of  the  distance  of  the 
object.  This  great  divergence  of  the  rays  entering  the  eye, 
is  the  chief  circumstance  in  which  the  most  perfect  painting 
must  still  differ  in  its  effect  upon  the  eye  from  a  natural  scene; 
for  while  in  nature  every  ohject  according  to  its  distance  is 
sending  rays  which  reach  the  eye  with  different  divergence, 
and  which,  therefore,  can  produce  distinct  images  on  the  retina 
at  any  one  time  only  of  the  objects  which  are  at  due  distance 
from  it,  the  rays  from  a  picture  which  is  a  single  plane  surface, 
come  from  every  part  with  the  same  divergence,  and  the  eye 
must  feel  a  disappointment  in  not  having  to  accommodate  its 
power  of  bending  to  the  different  distances  attempted  to  be 
portrayed  on  the  canvas.  It  might  be  expected  that  this  kind 
of  disappointment  would  be  more  felt  on  looking  at  a  common 
picture  placed  a  few  feet  from  the  eye,  than  at  the  sort  of  pic- 
ture called  panorama,  which  is  on  a  larger  scale  and  proportion- 
ately more  distant,  but  such  is  not  the  case;  and  the  reason 
seems  to  be,  that  in  the  former  the  illusion  is  not  intended  to 
be  complete,  the  fact  of  its  being  but  a  picture  not  being  at  all 
concealed,  and  the  eye  is  therefore  at  once  told  to  expect  a  dif- 
ference of  feeling; — but  in  the  panorama,  the  whole  circum- 
stances are  arranged  to  deceive  the  eye  entirely,  if  possible, 
and  to  make  it  believe  that  the  images  on  the  retina  are  formed 
by  light  from  the  objects  themselves;  then,  to  the  eye  really 
deceived  in  all  other  particulars,  the  non-accordance  with  na- 
ture in  this  one  is  strongly,  and  by  some  persons  even  painful- 
ly felt,  so  as  on  their  first  entering  the  place  to  cause  them 
headach  or  giddiness.  The  illusion,  and  consequently  the  plea- 
sure from  viewing  any  picture,  may  be  made  more  complete  by 
the  spectator  using  lenses  or  spectacles,  such  that  the  focal  dis- 
tance shall  be  equal  to  the  distance  of  the  painting  from  the 
eye;  because  such  lenses,  as  was  formerly  explained,  would 
render  all  the  rays  entering  the  eye  nearly  parallel,  and  there- 
fore very  nearly  such  as  would  arrive  from  objects  at  any  con- 
siderable distance. 


200  LIGHT. 

4th.    Convergence  of  the  axes  of  the  eye. 

This  is  the  last  circumstance  to  be  mentioned,  by  which  a 
person,  through  the  eye,  judges  of  the  distance  of  objects.  In 
consequence  of  there  being  in  the  two  eyes  corresponding 
parts,  which  must  be  similarly  affected  by  any  object,  that  the 
person  may  have  single  vision  of  it — as  was  explained  in  a 
former  page,  the  axes  of  both  eyes  must  point  to  the  object, 
and  if  it  happen  to  be  very  near,  they  will  meet  and  cross  each 
other  so  near  the  face  as  to  produce  the  appearance  of  squint- 
ing,— seen  when  a  person  tries  to  look  at  the  point  of  his 
nose, — but  if  the  object  be  more  distant,  the  obliquity  will  be 
less,  until  at  last  the  eyes,  directed  to  a  thing  at  a  very  great 
distance,  will  have  their  axes  almost  parallel.  The  last  figure 
may  serve  also  to  explain  this  subject.  Supposing  E  and  F  to 
mark  the  places  of  the  two  eyes,  if  the  object  looked  at  be 
near  them,  as  at  a,  they  must  be  very  much  turned  inwards, 
that  their  axes  may  meet;  if  it  be  at  b,  they  will  be  less  turned; 
if  at  c,  less  still,  and  so  forth. 

When  the  eyes  are  not  directed  to  any  thing  in  particular, 
the  axes  generally  become  parallel,  or  as  if  they  were  pointed 
to  a  very  distant  object:  and  because  this  happens  generally 
when  persons  are  reflecting  on  things  which  are  absent  and 
seen  only  by  the  mind's  eye,  it  is  an  expression  of  countenance 
held  to  mark  contemp-  ition  or  thoughtful  ness. 

The  direction  of  the  visual  axes  is  a  particular  like  the  diver- 
gence of  light,  as  to  which  a  mere  picture  can  never  produce 
upon  the  eye  precisely  the  effect  of  the  objects  themselves.  To 
see  a  picture,  the  axes  must  meet  at  a  few  feet  from  the  eye; — 
while  to  see  the  objects  of  nature,  they  often  do  not  meet  nearer 
than  at  miles.  By  a  glass,  however,  as  will  be  explained  a  lit- 
tle farther  on,  it  is  possible  to  correct  also  this  defect,  and  to 
render  an  optical  illusion,  as  reg-rds  still  objects,  absolutely 
complete. 

When  a  picture  has  to  represent  objects  supposed  far  from  the 
eye,  the  farther  the  picture  itself  is  placed  from  the  eye,  sup- 
posing the  figures  to  be  made  proportionately  large,  the  more 
nearly  perfect  will  the  illusion  become,  because  the  divergence 


THE  EYE — CONVERGENCE  OF  VISUAL  AXES.     .     •    201 

of  rays  and  convergence  of  the  axes  (the  two  circumstances  in 
which  the  effect  on  the  eye  of  a  mere  picture  must  always  dif- 
fer frgm  that  of  a  real  scene)  will  be  in  proportion  more  near- 
ly natural.  This  explains  in  part  why  the  picture  called  pano- 
rama (from  Greek  words,  signifying  a  view  of  all)  is  an  exhi- 
bition so  charming;  for  usually  the  painting  is  far  removed  from 
the  eye,  and  is  drawn  on  a  proportionately  large  scale,  and  the 
eyes  feel  that  the  light  comes  from  a  considerable  distance,  and 
that  their  axes  do  not  need  to  converge  very  much;  and  when, 
in  such  a  case,  the  first  impression  of  the  want  of  perfect  con- 
formity has  passed  away,  the  illusion  becomes  nearly  complete. 
But  a  not  less  important  peculiarity  in  the  panorama  is,  that  in- 
stead of  being  a  painting  on  a  plane  surface  like  common  pic- 
tures, and  embracing  only  a  small  part  of  the  field  of  view,  it 
is  on  a  surface  which  entirely  surrounds  the  spectator,  and  on 
which  all  the  objects,  visible  in  every  direction  from  the  sup- 
posed place,  are  seen  in  the  very  situations  which  in  nature  they 
hold;  and  the  spectator  is  enabled  to  conceive  much  more  dis- 
tinctly of  each  particular  by  seeing  it  in  relation  to  others 
around.  Few  persons  can  forget  the  impression  made  on  them 
by  the  first  panorama  which  they  may  have  seen;  and  with  in- 
creased maturity  of  judgment,  still  more  and  stronger  reasons 
are  discovered  for  admiring  this  miraculous  mode  of  instantly 
transporting  persons  to  any  distance,  beyond  seas  and  other  dan- 
gers, to  contemplate  at  their  ease  the  most  interesting  scenes  of 
nature,  represented  under  the  most  favourable  circumstances  of 
light,  season,  weather,  &c.  Hence,  few  persons  of  good  taste 
neglect  the  opportunity  now,  in  most  great  towns  so  frequently 
offered,  of  obtaining  at  so  little  cost  so  high  a  gratification. 

To  correct  the  slight  remaining  optical  defects  of  a  common 
panorama,  a  large  lens  may  be  used,  of  which  the  focal  distance 
is  the  distance  of  the  picture  from  the  eye.  This  has  the  effect 
of  converting  the  divergence  of  the  rays  of  light  into  the  paral- 
lelism which  belongs  to  the  supposed  remoteness  of  the  objects, 
and  it  also  bends  the  light  so  that  the  axes  of  the  eyes  become 
parallel.  The  author  has  found  a  convenient  mode  of  using  the 
lens  for  such  a  purpose  to  be,  to  cut  out  two  round  pieces  from 
opposite  sides  of  it,  and  to  form  them  into  a  pair  of  spectacles: — 


202    •     .  LIGHT. 

from  one  lens  three  pairs  may  be  formed. — Panorama  exhibiters 
should  always  keep  such  lenses  or  spectacles  for  the  use  of  vi- 
siters. 

The  effect  of  the  magnitude  and  distance  of  the  ordinary  large 
panoramic  views  might  be  had  with  the  assistance  of  proper 
glasses,  from  even  the  smallest  picture  or  engraving  embracing 
the  same  field;  and  it  is  remarkable  that  some  enterprising  per- 
son has  not  undertaken  to  publish  a  series  of  interesting  views 
fitted  to  be  used  in  that  way.  A  common  panorama  occupying 
a  circular  wall  of  150  feet  circumference  and  twenty  feet  high, 
might  be  reduced,  still  retaining  the  same  truth  of  proportions, 
to  appear  on  a  sheet  of  paper  five  feet  long  and  eight  inches 
high  or  broad-'-or  on  a  common  sheet  of  paper,  which  might 
afterwards  be  cut  into  three  stripes  to  be  joined  endways;  and 
if  this  paper  were  set  up  in  a  suitable  frame,  like  a  wall  round  the 
head  of  a  spectator,  while  its  edges  were  concealed  by  drapery 
or  otherwise,  and  the  eye  could  only  view  it  through  fit  glasses 
placed  in  its  centre  and  made  to  turn  round  so  as  to  command 
the  whole,  it  could  not  by  an  ordinary  spectator  be  distinguished 
from  the  large  panorama.  With  the  art  of  lithography,  now  so 
well  adapted  to  producing  soft  representations  of  scenery,  the 
expense  of  such  views  might  be  very  moderate,  allowing  them 
to  form  a  common  part  of  library  furniture.  When  we  reflect 
upon  the  expansion  of  mind  obtained  by  travelling,  and  that  not 
a  few  of  the  advantages  would  follow  a  familiarity  with  a  good 
selection  of  panoramic  views,  it  is  not  perhaps  too  much  to  sup- 
pose that  courses  of  instruction  on  geography,  history,  &c.  may 
before  long  be  illustrated  by  this  most  interesting  mode  of  aid- 
ing the  conception  and  memory. 

Common  paintings  and  prints  may  be  considered  as  parts  of 
a  panoramic  representation,  showing  as  much  of  that  general 
field  of  view  which  always  surrounds  a  spectator,  as  can  be  seen 
by  the  eye  turned  in  one  direction,  and  looking  through  a  win- 
dow or  other  opening.  The  pleasure  from  contemplating  these 
is  much  increased  by  using  a  lens  or  such  spectacles  as  above  de- 
scribed. There  is  such  a  lens  fitted  tip  in  the  shops,  with  the 
title  of  optical  pillar  machine,  or  diagonal  mirror,  and  the 
print  to  be  viewed  is  laid  upon  a  table  beyond  the  stand  of  the 


THE  EYE COSMORAMA.  203 

lens,  and  its  reflection  in  a  mirror  supported 
diagonally  over  it,  is  viewed  through  the  lens. 
The  illusion  is  rendered  more  complete  in 
such  a  case  by  having  a  box,  as  a,  b,  to  receive 
the  painting  on  its  bottom,  and  where  the 
lens  and  mirror,  fixed  in  a  smaller  box  above 
at  a,  are  made  to  slide  up  and  down  in  their 
place  to  allow  of  readily  adjusting  the  focal  distance.  This  box 
used  in  a  reverse  way  becomes  a  perfect  camera  obscura.  The 
common  show*stalls  seen  in  the  streets  are  boxes  made  some- 
what on  this  principle,  but  without  the  mirror;  and  although  the 
drawings  or  prints  in  them  are  generally  very  coarse,  they  are 
not  uninteresting.  To  children  whose  eyes  are  not  yet  very 
critical,  some  of  these  show-boxes  afford  an  exceeding  great 
treat. 

A  still  more  perfect  contrivance  of  the  same  kind  has  been 
exhibited  for  some  time  in  London  and  Paris  under  the  title  of 
Cosmorama  (from  Greek  words  signify  in  gw'e&j,?  of  the  world) 
because  of  the  great  variety  of  views.)  Pictures  of  moderate 
size  are  placed  beyond  what  have  the  appearance  of  common  win- 
dows, but  of  which  the  panes  are  really  large  convex  lenses  fit- 
ted to  correct  the  errors  of  appearance  which  the  nearness  of 
the  pictures  would  else  produce.  Then  by  farther  using  various 
subordinate  contrivances,  calculated  to  aid  and  heighten  the  ef- 
fects, even  shrewd  judges  have  been  led  to  suppose  the  small 
pictures  behind  the  glasses  to  be  very  large  pictures,  while  all 
others  have  let  their  eyes  dwell  upon  them  with  admiration,  as 
magical  realizations  of  the  natural  scenes  and  objects.  Because 
this  contrivance  is  cheap  and  simple,  many  persons  affect  to  de- 
spise it;  but  they  do  not  thereby  show  their  wisdom:  for  to  have 
made  so  perfect  a  representation  of  objects,  is  one  of  the  most 
sublime  triumphs  of  art,  whether  we  regard  the  pictures  drawn 
in  such  true  perspective  and  colouring,  or  the  lenses  which  as- 
sist the  eye  in  examining  them. 

It  has  already  been  stated,  that  the  effect  of  such  glasses  irt 
looking  at  near  pictures,  is  obtainable  in  a  considerable  degree 
without  a  glass,  by  making  the  pictures  very  large  and  placing 
them  at  a  corresponding  distance.  The  rule  of  proportion  in 


204  LIGHT. 

such  a  case  is,  that  a  picture  of  one  foot  square  at  one  foot  dis- 
tance from  the  eye,  appears  as  large  as  a  picture  ,of  60  feet 
square  at  60  feet  distance.  The  exhibition  called  th*e  Diorama 
is  merely  a  large  painting  prepared  in  accordance  with  the  prin- 
ciple now  explained.  In  principle  it  has  no  advantage  over  the 
cosmorama  or  the  show  box,  to  compensate  for  the  great  ex- 
pense incurred,  but  that  many  persons  may  stand  before  it  at  a 
time,  all  very  near  the  true  point  of  sight,  and  deriving  the 
pleasure  of  sympathy  in  their  admiration  of  it,  while  no  slight 
motion  of  a  spectator  can  make  the  eye  lose  its  point  of  view. 

A  round  building  of  prodigious  magnitude  has  lately  been 
erected  in  the  Regent's  Park  in  London,  on  the  walls  of  which 
is  painted  a  representation  of  London  and  the  country  around, 
as  seen  from  the  cross  on  the  top  of  St.  Paul's  Cathedral.  The 
scene  taken  altogether  is  unquestionably  one  of  the  most  extraor- 
dinary which  the  whole  world  affords,  and  this  representation 
combines  the  advantages  of  the  circular  view  of  the  panorama, 
the  size  and  distance  of  the  great  diorama,  and  of  the  details  being 
so  minutely  painted,  that  distant  objects  may  be  examined  by 
a  telescope  or  opera  glass. 

From  what  has  now  been  said,  it  may  be  understood,  that  for 
the  purpose  of  representing  still  nature,  or  mere  momentary 
states  of  objects  in  motion,  a  picture  truly  drawn,  truly  co- 
loured, and  which  is  either  very  large  to  correct  the  divergence 
of  light  and  convergence  of  visual  axes,  or,  if  small,  is  viewed 
through  a  glass,  would  affect  the  retina  exactly  as  the  realities. 
But  the  desideratum  still  remained  of  being  able  to  paint  mo- 
tion. Now,  this  too  has  been  recently  accomplished,  and  in 
many  cases  with  singular  felicity,  by  making  the  picture  trans- 
parent, and  throwing  lights  and  shadows  upon  it  from  behind. 
In  the  exhibitions  of  the  diorama  and  cosmorama  there  have 
been  represented  with  admirable  truth  and  beauty  such  pheno- 
mena as — the  sun-beams,  occasionally  interrupted  by  passing 
clouds,  and  occasionally  darting  through  the  windows  of  a  ca- 
thedral and  illuminating  the  objects  in  its  venerable  interior — 
the  rising  and  disappearing  of  mist  over  a  beautiful  landscape, — 
running  water,  as,  for  instance,  the  cascades  among  the  sublime 
precipices  of  Mount  St.  Gotharcl,  in  Switzerland ; — and,  most 


THE  EYE PAINTING  REPRESENTING  MOTION.  205 

surprising  of  all,  a  fire  or  conflagration.  In  the  cosmorama  of 
Regent  Street,  the  great  fire  of  Edinburgh  was  admirably  re- 
presented:— first,  that  fine  city  was  seen  sleeping  in  darkness, 
while  the  fire  began,  then  the  conflagration  grew  and  lighted  up 
the  sky,  and  soon,  at  short  intervals,  as  the  wind  increased,  or 
as  roofs  fell  in,  there  were  bursts  of  flame  towering  to  heaven, 
and  vividly  reflected  from  every  wall  or  spire  which  caught  the 
direct  light — then  clouds  of  smoke  were  seen  rising  in  rapid 
succession  and  sailing  northward  upon  the  wind,  until  they  dis- 
appeared in  the  womb  of  distant  darkness.  No  one  can  have 
viewed  that  appalling  scene  with  indifference,  and  the  impres- 
sion left  by  the  representation,  on  those  who  knew  the  city,  can 
scarcely  have  been  weaker  than  that  left  on  those  who  saw  the 
reality.  The  mechanism  for  producing  such  effects  is  very  sim- 
ple; but  spectators,  that  they  may  fully  enjoy  them,  need  not 
particularly  inquire  about  it. 

It  is  remarkable,  when  the  imagination  is  once  excited  by 
some  beautiful  or  striking  view,  how  readily  any  visual  hint 
produces  clear  and  strong  impressions.  One  day  in  the  cosmo- 
rama, a  school  boy  visiter  exclaimed  with  fearful  delight,  that 
he  saw  a  monstrous  tiger  coming  from  its  den  among  the  rocks; 
it  was  a  kitten  belonging  to  the  attendant,  which  by  accident 
had  strayed  upon  the  paintings.  And  another  young  spectator 
was  heard  calling  that  he  saw  a  horse  galloping  up  the  moun- 
tain-side; it  was  a  minute  fly  crawling  slowly  along  the  canvas. 
There  is  in  this  department  a  very  fine  field  yet  open  to  the  ex- 
ercise of  ingenuity,  for  the  contemplation  of  pictures  represent- 
ing the  motion  or  progressive  events,  may  be  made  the  occasion 
of  mental  excitement  the  most  varied  and  intense.  For  in- 
stance, there  are  few  scenes  on  earth  calculated  to  awaken  more 
interesting  reflections  on  the  condition  of  human  nature  than 
that  beheld  by  a  person  who  sails  along  the  river  Thames  from 
London  to  the  sea,  a  distance  of  about  sixty  miles,  through  the 
wonders  which  on  every  side  there  crowd  on  the  sight — the  fo- 
rest of  ships  from  all  parts  of  the  world — the  glorious  monu- 
ments of  industry,  of  philanthropy,  of  science — the  marks  of 
the  riches,  the  high  civilization,  and  the  happiness  of  the  peo- 
ple. Now,  this  scene  was  last  year  in  one  of  our  theatres  stri- 


206  LIGHT. 

kingly  portrayed  by  what  was  called  a  moving  panorama  of 
the  southern  bank  of  the  Thames.  It  was  a  very  long  painting, 
of  which  a  part  only  was  seen  at  a  time  gliding  slowly  across 
the  stage,  and  the  impression  made  on  the  spectators  was,  that 
of  their  viewing  the  realities  while  sailing  down  the  river  in  a 
steam  boat.  In  the  same  manner  the  whole  coast  of  Britain 
might  be  most  interestingly  represented — or  any  other  coast,  or 
any  line  of  road,  or  even  a  line  of  balloon  flight.  There  was 
another  moving  panorama  exhibited  about  the  same  time  at 
Spring  Gardens,  aiming  at  an  object  of  still  greater  difficulty, 
viz.  to  depict  a  course  of  human  life;  and  the  history  chosen, 
was  that  of  the  latter  part  of  Buonaparte's  career.  Scenes  re- 
presenting the  principal  events  were,  in  succession,  and  appa- 
rently on  the  same  canvas,  made  to  glide  across  the  field  of 
view,  so  designed  that  the  real  motion  of  the  picture  gave  to 
the  spectator  the  feeling  of  the  events,  being  only  then  in  pro- 
gress, and  with  the  accompaniments  of  clear  narration  and  suit- 
able music,  they  produced  on  those  who  viewed  them  the  most 
complete  illusion.  The  story  began  with  the  blow  struck  at 
Buonaparte's  ambition  in  the  battle  of  Trafalgar,  and  to  mark 
how  completely,  by  representations  of  various  moments  and  si- 
tuations of  the  battle,  the  spectators  were  in  imagination  made 
present  to  it,  the  author  of  this  work  may  mention,  that  on  the 
occasion  of  his  visiting  the  exhibition,  a  young  man  seeing  a 
party  of  British  preparing  to  board  an  enemy's  ship,  started 
from  his  seat  with  a  hurra,  and  seemed  quite  surprised  when 
he  found  that  he  was  not  really  in  the  battle.  To  the  first  views 
there  succeeded  many  others,  similarly  introduced  and  ex- 
plained, in  each  of  which  the  hero  himself  appeared: — there 
were,  his  defeat  at  Waterloo — his  subsequent  flight — his  delive- 
ry of  himself  to  the  British  Admiral — his  appearing  at  the 
gangway  of  the  Bellerophon  to  thousands  of  spectators,  wait- 
ing in  boats  around,  while  he  was  in  Plymouth  Harbour,  pre- 
vious to  his  departure  for  ever  from  the  shores  of  Europe — his 
house  and  habits  during  his  exile,  with  various  views  of  St. 
Helena: — and,  last  of  all,  that  solemn  procession,  in  which  the 
bier  with  his  lifeless  corpse  appeared  moving  slowly  on  its  way 
to  the  grave  under  the  willow  tree.  The  exhibition  now  spoken 


THE  EYE — HISTORICAL  PAINTING.  207 

of  might  have  been  made  much  better  in  all  respects,  yet  in  its 
mediocrity  it  served  to  prove  how  admirably  adapted  such 
unions  of  painting,  music,  and  narration,  or  poetry,  are  to  af- 
fect the  mind,  and  therefore  to  become  the  means  of  conveying 
most  impressive  lessons  of  historical  fact  and  moral  principle. 

Painting,  whether  employed  to  portray  scenes  of  entirely 
still  nature,  or  scenes  involving  some  kind  of  motion  as  above 
described,  has  still  as  its  great  aim  or  end  merely  to  represent 
interesting  subjects,  and  to  give  to  the  spectator  as  much  as  pos- 
sible the  clear  conception  of  them,  which  is  obtained  by  ocu- 
lar examination  of  realities.     Painting  thus,  as  a  system  of  vi- 
sual signs  of  thought,  becomes  like  language,  which  is  a  system, 
of  audible  signs,  a  means  of  expanding  the  boundaries  of  indi- 
vidual human  existence  into  wider  space  and  time,  and  thus  of 
ennobling  human  nature.    While  it  portrays  only  strict  matters 
of  fact,  whether  of  past  or  present  time,  as  particular  human  in- 
dividuals, objects  of  natural  history,  the  beautiful  and  magnifi- 
cent scenes  of  nature,  interesting  events  which  the  artist  had 
the  means  of  faithfully  representing,  &c.,  it  may  be  called  truly 
historical  painting,  embodying  the  materials  of  true  history, 
both  natural  and  civil,   and  then  is  of  singular  value.      But 
even  when  applied  to  other  purposes,  it  may  still  be  fraught  with 
delight;  and  just  as  language,  of  which  the  grand  object  or  use 
is  to  express  strict  truths,  has  still  been  admirably  employed  in 
giving  a  permanent  existence  to  a  variety  of  fictions,  from  the 
wildest  fables  and  rhapsodies  to  the  historical  plays  and  novels 
of  modern  times,  as  those  of  Shakspeare  and  of  Scott — which 
plays  and  novels,  although  not  furnishing  true  portraits  of  indi- 
vidual human  nature,  are  yet  most  correct  portraits  of  general 
human  nature — so  may  painting  be  employed  in  embodying  fic- 
tions adapted  to  its  peculiar  powers,  and  so  as  to  prove  the  ar- 
tist endowed  with  the  highest  degree  of  human  genius.     It 
should  always  be  recollected,  however,  that  what  is  usually  dig- 
nified with  the  name  of  historical  painting,  really  bears  to  his- 
torical truth  only  the  kind  of  relation  which  novels  and  plays 
bear  to  it,  and  often  approaches  much  less  nearly  to  the  truth: 
for  it  pretends  to  relate  a  thousand  minute  circumstances  which 
no  history  has  preserved,  and  which,  therefore,  only  the  imagi- 


208  LIGHT. 

nation  of  the  artist  can  supply.  Thus,  when  a  painter,  know- 
ing that  Lucretia  stabbed  herself  in  the  presence  of  her  father 
and  others,  after  the  crime  of  Tarquin,  exhibits  a  woman  dying, 
and  a  certain  number  of  persons  around  her  in  horror  and  as- 
tonishment, he  no  more  represents  the  real  Lucretia  and  her 
friends  than  he  represents  any  other  particular  young  woman  and 
her  friends;  for  he  is  quite  assured  that  not  one  of  the  figures 
in  such  a  picture  is  a  portrait  of  the  individual  whose  name 
it  bears:  his  picture,  therefore,  in  so  far  is  an  untruth  or  fiction, 
while  it  very  probably  shares  the  additional  errors  and  even  ab- 
surdities so  common  among  historical  painters — errors,  for  in- 
stance, in  respect  of  national  usage  in  costume,  religion,  man- 
ners, &c.,  and  in  respect  to  general  appearance,  as  when  a  Reu- 
bens, wishing  to  represent  Sabine  or  other  ladies,  gave  them  the 
degree  of  corpulency  deemed  comely  in  his  own  country,  al- 
though it  notably  contrasted  with  the  true  forms  of  Italian  or 
Grecian  nymphs.  From  all  this  it  appears  that  historical  pic- 
tures may  often  be  regarded  as  portraitures,  not  of  the  realities, 
but  of  comedians  acting  scenes  in  historical  plays  intended  to 
represent  the  realities. 

In  dealing  with  the  events  of  ordinary  history,  there  is  no 
strong  reason  why  artists  may  not  please  themselves  and  their 
spectators  as  we  have  now  been  describing;  but  it  may  admit  of 
doubt  whether  similar  liberties  should  be  allowed  with  respect 
to  religion.  Yet  any  painting  of  the.  last  supper,  for  instance, 
or  of  the  ascension,  is  not  likely  to  be  more  true  than  a  thea- 
trical representation.  To  judge  of  the  nature  of  such  a  picture, 
we  have  only  to  suppose  any  of  the  events  recorded  in  the  New 
Testament  to  be  represented  by  a  painter  in  China  with  the 
countenances  such  as  are  seen  on  Chinese  tea-boxes;  such  a  re- 
presentation would  appear  in  Europe  revoltingly  absurd;  but 
the  common  practice  here  is  only  a  degree  better;  as  twenty 
painters  undertaking  to  treat  the  same  subject,  will  put  different 
persons  into  all  the  situations.  Then  it  can  produce  no  pleasing 
impression  on  a  Christian's  mind  to  be  told,  that  an  admired 
painting  of  the  crucifixion  was  made  chiefly  from  the  body  of 
an  executed  murderer;  or  that  for  a  praised  representation  of 
the  triumphal  entry  into  Jerusalem,  the  painter  had  deemed  his 


THE  EYE — ART  OF  PAINTING.  209 

own  physiognomy  the  most  befitting  for  the  principal  figure, 
while  he  copied  the  portrait  of  a  noted  modern  sceptic  as  a  spe- 
cimen of  the  had  men,  of  an  equally  noted  believer  as  a  speci- 
men of  the  good,  while  wife,  cousins,  acquaintances,  and  old 
clothes-men,  served  to  make  up  the  remaining  groups.  With 
the  knowledge  that  such  things  have  often  been,  it  need  not  sur- 
prise, that  many  persons  of  correct  feeling  turn  with  horror  from, 
all  these  mimicries  and  falsehoods,  to  seek  their  idea  of  God 
and  his  providence  in  the  sublime  descriptions  of  his  attributes, 
which  written  language  conveys,  and  which  all  creation,  in  a 
mute  language,  scarcely  less  impressive,  so  strongly  confirms. 
When  men  generally  could  not  read,  and  as  a  mass  were  ex- 
tremely ignorant,  various  means  of  fixing  their  attention  upon 
religious  subjects  might  be  useful,  and  therefore  proper,  as  sa- 
cred plays,  certain  processions,  pictures,  &c.,  which  have  now 
in  many  countries  ceased  to  be  either;  but  a  person  of  good 
sense  will  continue  to  regard  with  a  certain  respect  whatever  at 
any  time  may  have  contributed  to  reclaim  portions  of  mankind 
from  barbarism  and  wickedness,  to  the  just  appreciation  of  the 
divine  charities  of  a  pure  religion. 

There  are  in  painting  other  classes  of  fictions,  which  pretend 
to  nothing  beyond  fiction,  and  which  are  yet  truly  admirable: 
such  are  personifications  of  the  virtues  and  vices,  serving  to  re- 
commend the  practice  of  the  former,  and  to  deter  from  that  of 
the  latter; — almost  all  Hogarth's  works  are  of  this  character, 
and  evince  the  highest  mental  acumen  and  genius: — then  may 
be  mentioned  the  personifications  of  what  have  been  called  the 
elements  and  powers  of  nature,  including  many  of  the  person- 
ages of  the  Heathen  mythology — then  other  generalizations  of 
the  characteristics  of  human  or  other  nature,  as  scenes  of  do- 
mestic affection,  of  the  play  of  the  passions,  &c.  &c. ;  and  be- 
cause many  subjects  when  so  sketched,  are  intelligible  to  the 
eye  with  the  suddenness  of  lightning,  where  longest  verbal  de- 
scription would  convey  the  idea  but  imperfectly,  the  art  of 
painting  in  regard  to  them  possesses  a  truly  magical  and  inesti- 
mable power. 

As  painting,  whether  engaged  in  representing  matters  of  fact 
or  of  fiction,  can  accomplish  its  ends  only  through  the  arts  of 

27 


210  LIGHT. 

drawing  or  linear  perspective,  and  of  shading  and  colouring, 
or  aerial  perspective,  these  subjects  require  to  be  studied  by 
every  artist  with  great  attention;  but  it  is  important  for  all  to 
be  aware  that  the  greatest  mastery  over  these,  which  are  mere- 
ly the  mechanical  parts  of  the  art,  will  go  a  very  short  way  to- 
wards producing  good  performances,  unless  there  be  present 
also  the  genius  to  select  or  to  compose  subjects  worthy  of  being 
represented.     The  latter  remark  seems  the  more  necessary,  be- 
cause there  is  in  human  nature  a  disposition  to  value  so  much 
the  means  by  which  important  ends  are  attained,  that  often  the 
end  itself  is  forgotten  in  the  contemplation  of  the  means, — 
while  among  painters,  as  among  persons  of  other  occupations, 
the  talent  for  the  inferior  or  more  mechanical  departments  of 
the  art,  is  more  common  than  for  the  higher.     It  is  hence,  that 
the  subordinate  accomplishments  of  the  painter  are,  by  not  a 
few,  both  artists  and  pretended  connoisseurs,  supposed  to  be  the 
principal.     But  this  is  evidently  to  value  the  dress  or  clothing, 
instead  of  the  person,  as  might  be  observed  of  the  criticism  of 
a  mere  tailor  or  dress-maker  on  a  courtly  assembly  of  indivi- 
duals distinguished  by  rank  and  talent.     The  same  very  com- 
mon error,  arising  from  the  weakness  or  short-sightedness  of 
ordinary  human  nature,  of  confounding  means  with  ends,  is 
strikingly  exemplified  in  the  case  of   the  person,  who,  per- 
ceiving that  money  will  procure  all  desirable  things,  at  last  be- 
comes the  insane  miser,  and  dies  from  want  of  the  common  ne- 
cessaries of  life  rather  than  touch  his  hoarded  treasures: — it  is 
seen  also  in  the  case  of  the  mere  pedagogue,  who  values  the  ab- 
stractions of  language  or  grammar,  and  of  mathematics  more 
than  the  useful  knowledge  which  they  may  serve  to  convey; 
or,  lastly,  still  more  suitably  for  our  purpose,  in  the  case  of  the 
bibliomaniac,  who  regards  the  type  and  binding  of  his  books 
more  than  the  subject.      To  prove  how  unessential  what  is 
called  high-finishing  in  painting,  is  to  the  complete  obtainment 
of  the  purposes  of  the  art,  we  have  the  cartoons  of  the  immor- 
tal Raphael,  which  to  the  mere  mechanic  appear  almost  to  be 
daubs:  and  many  of  the  mere  sketches  of  genius  are  to  a  true 
taste  more  precious  than  some  of  the  most  finished  pieces  in 
our  galleries.     Again,  of  what  consequence  is  it  to  a  man  whe- 


THE  TELESCOPE.  211 

ther  he  see  approaching  the  friend  of  his  heart  by  day-light  or 
candle  light,  or  with  the  source  of  light  above  or  below,  &c., 
provided  there  is  light  enough  for  him  to  distinguish  clearly  the 
friend  of  his  heart.  A  painter  will  discover  the  difficulties 
which  a  brother  artist  had  to  surmount  in  representing  an  ob- 
ject in  some  particular  predicament,  as  regards  the  light,  &c., 
and  may  estimate  the  talent  accordingly;  but  the  great  mass, 
even  of  the  most  accomplished  ordinary  spectators,  will  gene- 
rally be  looking  beyond  the  sign  to  the  thing  signified;  and 
perhaps  few  will  at  all  heed  the  difficulties.  In  consequence  of 
the  prejudice  in  favour  of  "  a  sweet  or  adorable  bit  of  colour- 
ing," as  it  will  sometimes  be  called,  and  which  in  truth  may 
have  the  merit  of  most  natural  colouring,  there  are  preserved 
in  many  galleries,  pictures  disgusting  in  almost  all  other  re- 
spects,— as  of  drunken  Dutch  boors,  with  fiery  noses  and  phy- 
siognomies degrading  to  human  nature,  &c.  &c.;  on  seeing  which 
the  man  of  taste  deplores  that  the  art  of  representing  should  be 
prostituted  to  the  vile  purpose  of  representing  things  of  worse 
than  no  interest,  and  that  its  nobler  objects  should  be  so  often 
forgotten. 

ee  Whan  the  image  formed,  as  above  described,  beyond  a  lens 
is  viewed  in  the  air  by  an  eye  placed  still  farther  beyond 
in  the  same  direction,  the  arrangement,  according  to  mi- 
nor circumstances,  constitutes  either  the  common  TELES- 
COPE or  MICROSCOPE."  (Read  the  whole  second  paragraph 
of  the  Analysis,  page  122.) 

The  name  of  telescope  (a  compound  Greek  term,  signifying 
to  see  far,  as  microscope  signifies  to  see  what  is  small,)  applies 
to  that  wondrous  instrument  of  modern  invention  by  which  the 
intelligent  soul  on  the  beams  of  light  as  its  path,  is  enabled  as 
it  were,  to  dart  widely  into  space  for  the  purpose  of  contem- 
plating the  distant  glories  of  creation;  or,  again,  by  which  dis- 
tant objects  are  instantly  brought  near  to  the  eye  for  the  purpose 
of  convenient  inspection.  In  ancient  times,  a  man,  while  look- 
ing with  admiration  on  the  bright  face  of  the  moon,  might  have 
exclaimed,  "  Would  that  I  had  the  power  to  fly  upwards  to  that 
celestial  orb,  the  better  to  understand  its  nature  and  beauties;" 


212  LIGHT. 

but  he  could  little  have  dreamed  that  the  day  was  coming  when 
human  ingenuity  would  in  effect  achieve  the  wish: — now  by  the 
telescope  it  is  achieved;  for,  an  instrument  which  merely  dou- 
bles apparent  magnitudes,  shows  the  moon  exactly  as  she  would 
appear  to  a  person  who  had  ascended  toward  her  from  the  earth 
a  distance  of  120,000  miles,  while  one  of  greater  power  pro- 
duces effects  correspondingly  great.  But  to  examine  the  hea- 
venly bodies  is  by  no  means  the  only  use  of  the  telescope,  man 
being  often  more  concerned  to  discover  what  is  passing  on  the 
surface  of  the  earth  around  him.  Thus,  by  a  telescope  the  military 
chief  descries  approaching  friends  or  foes,  while  yet  concealed 
from  others  and  from  the  naked  eye,  in  the  blue  mist  of  distant 
mountain  or  plain — or  similarly,  the  sailor,  whose  attention  has 
been  attracted  to  a  little  speck  on  the  sea  horizon,  discovers 
there  a  ship  of  class  and  nation  at  once  evident  to  him,  and  with 
the  crew  of  which,  by  the  additional  use  of  signal  flags,  he  is 
enabled  readily  to  converse; — at  midnight  a  telescope  directed 
to  a  distant  cathedral  will  so  effectually  call  it  into  the  presence 
of  the  observer,  that  on  the  clock  turret  may  be  read  from  the 
slow-moving  hands  the  unceasing  lapse  of  time.  A  man,  in  the 
midst  of  a  wide  plain,  or  on  a  lofty  hill-top,  or  far  on  the  face 
of  a  lake,  nay,  even  in  his  own  garden,  or  in  his  house  near 
some  open  window,  who  might  suppose  himself  quite  alone  and 
unseen,  would  yet  by  a  telescope  be  instantly  placed  under 
the  observation  of  whoever  chose  to  watch  him.  Some  remark- 
able cases  of  actions,  imagined  by  the  parties  to  have  been  done 
in  perfect  secrecy,  have  thus  been  brought  to  light. 

Now,  the  telescope  with  its  extraordinary  powers  exhibits  but 
another  modification  of  the  simple  case  described  at  page  144, 
and  exemplified  in  the  camera  obscura,  &c.,  of  an  image  formed 
for  visual  inspection  beyond  a  lens.  And  we  shall  here  ex- 
plain that  its  powers  depend  altogether  on  the  two  circum- 
stances; first,  of  its  large  lens  collecting  for  the  formation  of  the 
image  (subsequently  transferred  to  the  observer's  retina)  a  thou- 
sand or  more  times  the  quantity  of  light  which  the  naked  pu- 
pil could  receive;  and,  second,  of  its  forming  by  this  light  an 
image,  which  to  the  eye  brought  near  it  appears  vastly  larger 
than  the  object  itself  appears. 


THE  TELESCOPE.  213 

To  understand  this  well,  we  must  recal,  that  the  nature  of 
the  bending  of  light  in  passing  through  a  lens  is  such,  that  all 
the  rays  reaching  the  lens  from  any  point  of  a  visible  object  in 

front,  and  form- 
ing  what  is  called 
a  pencil  of  light, 
— as  that  spread- 
ing from  the  point  A  of  the  cross  here  represented  to  the  lens 
L — are  collected  in  a  corresponding  point,  as  r/,  at  the  focal 
distance  beyond  the  lens,  and  so  as  always  to  meet  the  central 
ray  of  the  pencil:  and  therefore  when  the  light  comes  from 
above  the  centre  of  the  lens,  the  focal  meeting  is  below,  as 
shown  here;  and  when  it  comes  from  below,  the  meeting  is 
above:  then  the  same  happening  as  regards  every  visible  point 
of  the  object  (the  rays  from  only  the  two  extreme  points  A 
and  B  are  here  represented)  at  corresponding  points  beyond  the 
lens  in  the  space  between  a  and  b,  the  collected  light,  if  re- 
ceived on  a  white  screen  placed  there,  as  in  the  camera  obscu- 
ra,  will  make  apparent  to  an  eye  in  any  direction  a  beautiful  in- 
verted image  of  the  object.  Now,  in  the  place  where  the  rays 
meet  to  form  this  image,  if  no  screen  be  interposed,  the  rays  are 
not  lost  or  destroyed,  but  merely  cross  each  other  in  the  air 
nearly  as  they  previously  crossed,  without  interference,  in  the 
lens,  and  spread  again  beyond  the  focal  points,  or  towards  c}  as 
here  shown,  as  they  originally  spread  from  the  several  points  of 
the  object  itself:  an  eye  therefore  placed  any  where  beyond 
c,  must  receive  portions  of  the  pencils  from  every  point  of 
the  image,  and  may  see  it  in  the  air  as  an  object  situated 
in  the  focus  of  the  lens.  This  may  be  observed  at  once  by 
holding  a  spectacle  glass  or  any  lens  at  proper  distance  be- 
tween an  object  and  the  eye. 

Now,  a  telescope  is  merely  a  long  tube,  blackened  within  to 
exclude  and  destroy  useless  light,  and  having  a  large  lens  called 
the  object  glass,  filling  one  end  of  it  as  a  window,  to  gather 
the  light  from  the  objects  in  front,  and  to  form  with  it  images 
near  the  other  end  of  the  tube,  where  they  may  be  convenient- 
ly inspected.  These  images,  for  a  purpose  to  be  immediately 
explained,  are  examined  through  another  lens  called  the  eye 


LIGHT. 

glass,  which  is  fixed  in  a  smaller  tube  made  to  slide  backwards 
and  forwards  in  the  larger,  so  as  to  admit  of  the  focal  distances 
being  adjusted  to  the  power  of  different  eyes,  &c.     The  accom- 
panying   sketch 

•p  j,  _____    s^ows   the    Pro' 

gress  of  the  light 
from  the  object 
A,  through  the 
object  glass  L,  to  form  an  image  at  b  a,  and  afterwards  to  be 
bent  by  the  eye  glass  D,  so  as  to  enter  the  pupil  of  the  eye  at 
E,  to  form  the  last  image  on  the  retina. 

In  the  simple  telescope,  with  only  two  lenses,  as  above  re- 
presented, called  the  astronomical  telescope,  the  image  is  in- 
verted: but  this  is  a  circumstance  of  no  importance  in  viewing 
the  heavenly  bodies;  to  fit  the  instrument,  however,  for  view- 
ing terrestrial  objects,  it  is  necessary  to  place  in  the  tube  ano- 
ther simple  or  compound  lens,  which  shall  form  a  second  image 
from  the  first,  and  by  inverting  a  second  time,  shall  produce  an 
image  really  upright. 

To  determine  how  much  larger  an  object  will  appear  when 
viewed  through  a  certain  telescope,  for  instance,  one  with  an 
object  glass  of  three  feet  focus,  than  when  viewed  by  the  naked 
eye,  we  must  recollect  that  the  image  is  formed  in  the  focus  of 
the  object  glass,  or  at  b  a,  and  subtends  from  the  centre  of  that 
lens,  as  at  c,  the  same  visual  angle  as  the  object  itself,  (a  fact  ex- 
plained at  page  152,)  and  to  an  eye  placed  there,  would  appear 
of  the  same  size  as  the  object;  but  if  the  eye  can  be  brought 
fifty  times  nearer  to  the  image,  this  will  appear  fifty  times  taller 
and  broader,  and  therefore  with  2,500  times  the  surface,  and 
thus,  as  compared  with  the  object,  may  be  called  much  magni- 
fied. Now,  as  the  naked  eye  cannot  see  distinctly  an  object 
nearer  to  it  than  at  about  six  inches,  because  of  the  great  diver- 
gence of  light  from  a  nearer  radiant  point,  the  telescope  in 
question,  without  an  eye  glass,  would  allow  the  eye  to  come 
only  six  times  nearer  to  the  image  than  when  at  the  centre  of 
the  object  glass,  and  would  only  magnify  the  diameter  six  times; 
but  if  then  an  eye  glass,  as  D,  of  half  an  inch  focus,  were 
placed  half  an  inch  from  the  image,  so  as  to  render  the  rays  of 


THE  TELESCOPE.  215 

ever)'-  pencil  parallel,  and  therefore  fitted  to  the  powers  of  the 
eye,  while  the  different  parcels  would  cross  each  other  a  little 
way  beyond  the  glass,  as  shown  above,  an  eye  placed  to  receive 
in  its  pupil  the  crossing  parcels,  would  see  the  image  as  large  as 
if  at  half  an  inch  from  it,  and  therefore  72  times  nearer  than  if 
viewed  from  the  object  glass,  and  therefore  again  as  of  72  times 
greater  diameter.  Now,  as  in  all  cases,  the  image  in  a  telescope 
is  in  the  focus  both  of  the  object  glass  and  eye  glass,  and  is 
therefore  nearer  to  the  latter  than  to  the  former  in  proportion 
as  their  focal  distances  differ,  the  magnifying  power  is  mea- 
sured by  that  difference — in  the  case  at  present  supposed,  the 
difference  is  as  72  to  1,  and  72  is  the  magnifying  power  of  the 
telescope.  The  rule  is  generally  thus  expressed:  "divide  the 
focal  distance  of  the  object  glass  by  that  of  the  eye  glass,  and 
the  quotient  is  the  magnifying  power."  It  is  always  to  be  re- 
membered, that  if  the  diameter  of  an  object  be  magnified  ten 
times,  the  surface  is  magnified  100  times,  and  so  in  proportion 
for  other  numbers. 

With  such  means  of  aiding  the  sight,  then,  is  it  that  we  dis- 
cover the  mountains  of  our  moon,  and  can  even  measure  their 
altitudes;  that  we  can  see  the  four  beautiful  moons  of  the  planet 
Jupiter;  that  we  can  perceive  marks  and  irregularities  on  the 
surfaces  of  the  other  planets,  enabling  us  to  say  at  what  rate 
they  severally  whirl  round  their  axes,  experiencing  the  pheno- 
mena of  day  and  night;  and  that  we  can  determine  many  other 
interesting  particulars. 

The  discovery  of  the  telescope  is  said  to  have  been  first  made 
accidentally  by  the  children  of  a  Dutch  spectacle  maker,  while 
they  were  playing  with  their  father's  work;  but  it  was  turned 
to  no  use  until  Galileo,  led  by  science,  fell  upon  it  again,  and 
with  the  knowledge  of  its  worth,  obtained  from  it  the  most  sub- 
lime results.  If  ever  human  heart  throbbed  with  delight,  it 
must  have  been  when  Galileo  first  directed  his  optic  tube  to  the 
heavens,  and  through  it  contemplated  so  many  glorious  objects 
which  no  human  eye  before  had  seen! — as  Venus,  our  beautiful 
morning  and  evening  star,  appearing  not  a  circle,  but  a  crescent, 
like  our  moon  in  her  quarters — as  the  satellites  of  Jupiter — the 
rings  of  Saturn — myriads  of  stars,  until  then  invisible  to  man; 


216  LIGHT. 

and,  in  a  word,  when  he  beheld  the  undoubted  proofs  of  the 
true  system  of  the  universe,  as  his  genius  had  before  conceived 
it,  uniting  the  greatest  simplicity  with  its  unspeakable  gran- 
deur. 

The  Galilean  telescope  was  simply  a  large  object  glass  to  col- 
lect much  light,  with  a  small  concave  eye  glass  placed  so  as  to 
intercept  the  converging  rays  before  they  reached  their  focus, 
and  to  change  their  convergency  into  the  parallelism  which  the 
eye  could  command.  This  telescope,  although  magnifying  less 
than  that  made  of  two  convex  glasses,  as  above  described,  still 
from  occasioning  no  loss  of  light  by  the  crossing  of  rays  in 
forming  an  image,  was  of  considerable  power.  The  same  prin- 
ciple is  now  adopted  for  the  common  opera  glass. 

It  was  explained  at  page  143,  that  a  ray  of  light,  in  being 
bent  or  refracted  by  transparent  media,  as  by  a  lens,  is  also  di- 
vided into  rays  of  the  different  colours  seen  in  the  rainbow. 
Hence,  an  image  formed  behind  a  simple  lens  has  coloured 
edges  or  fringes.  This  fact  rendered  the  images  of  small  ob- 
jects, formed  in  the  first  telescopes,  rery  indistinct;  and,  but 
for  the  important  discovery  originally  made  by  Dollond  the 
optician,  that  different  kinds  of  glass  have  the  dispersive  and 
refractive  powers  with  different  relations,  so  that  a  concave 
lens  of  a  certain  curve  applied  to  a  convex  lens  might  com- 
pletely counteract  the  dispersion  of  colour  by  the  latter,  while 
it  left  enough  of  the  convergence  of  the  rays  for  the  formation 
of  an  image — refracting  telescopes  would  have  always  been 
very  imperfect.  Dollond  called  his  telescopes  achromatic,  or 
without  colouring  power.  It  is  very  remarkable,  that  he  had 
the  fortune  to  obtain  some  glass  for  his  purposes  more  suitable 
than  any  which  has  been  procured  since,  or  which  could  be 
made  by  known  rules,  until  the  late  improvements  in  the  ma- 
nufacture suggested  by  the  ingenuity  of  Mr.  Farraday.  The 
author  carried  abroad  with  him  a  small  Galilean  telescope  of 
Dollond 's,  which  often  gave  more  correct  information  respect- 
ing minute  coloured  objects  at  a  distance,  as  signal  flags  at  sea, 
than  much  larger  glasses  of  modern  make. 

The  MICROSCOPE,  of  greatest  power,  and  with  the  form  called 
compound,  in  its  structure  approaches  very  closely  to  the  tele- 


THE  MICROSCOPE.  ^17 

scope,  the  chief  difference  being,  that  while  in  the  telescope  a 
large  distant  object  forms  in  the  focus  of  the  object  glass,  an 
image  exactly  as  much  smaller  than  itself  as  the  distance  of 
the  image  from  the  glass  is  Jess,  in  the  microscope,  conversely, 
a  small  object  placed  near  the  focus  of  the  object  glass  pro- 
duces a  more  distant  image,  as  much  larger  than-  itself  as  the 
image  is  more  distant, — -and  in  the  one  case  as  in  the  other, 
the  image  is  viewed  through  an  appropriate  eye  glass.  The 
object  glass  in  the  telescope  is  large,  in  the  microscope  it  is 
generally  very  small.  If,  .in  the  latter,  an  object  glass  be  used 
of  one-eighth  of  an  inch  focal  distance,  and  the  object  be  so 
placed  that  its  image  is  formed  at  six  inches,  the  image  will  be 
of  diameter  48  times  as  great  as  the  object,  or  will  have  near- 
ly 2,500  times  as  much  surface;  and  if  that  image  be  viewed' 
through  an  eye  glass  of  half  an  inch  focus,  the  image  will  ap- 
pear still  twelve  times  larger,  or  30,000  times  larger  than  the 
object. 

One  convex  lens  is  called  a  single  microscope,  and  it  mag- 
nifies, as  already  explained,  chiefly  by  allowing  the  eye  to  be 
brought  so  much  nearer  to  the  object  than  would  be  possible 
to  see  it  without  the  glass:  but  even  if  the  distance  of  the  eye 
and  object  is  not  changed,  a  lens  interposed  will  still  magnify 

by  bendiiii;  the  light,  as  at  d, 

-j  <;  J  »  _    »       >  7 

and  making  that  which  comes 
to  the  eye  at  c,  from  the  top 
.of  such  an  object,  as  the  little 
cross,  a,  appear  to  come  from 

b,  and  that  from  the  bottom  to  come  from  c,  thus  magnifying 
the  cross  here  represented  by  the  black  lines,  to  appear  of  the 
size  represented  by  the  dotted  lines.  A  concave  lens  minifies 
for  the  contrary  reason. 

Perhaps  there  is  not  a  greater  treat  for  a  person  who  has 
feeling  for  the  beauties  of  nature,' than  to  explore  with  the  mi- 
croscope. While  the  telescope  lifts  the  mind  to  the  contem- 
plation of  boundless  space,  occupied  by  myriads  of  suns,  and 
exhibits  this  globe  of  ours  as  less,  compared  with  the  universe 
around  it,  than  a  leaf  is  compared  with, a  forest,  or  a  grain  of 
sand  compared  with  alf  which  lies  on  ocean's  shore;  the  mi- 


2  IS 


LIGHT. 


croscope,  again,  excites  new  astonishment  by  showing  on  a  leaf, 
or  in  a  single  drop  of  water  in  which  the  leaf  has-been  infused, 
thousands  of  living  creatures,  and  of  creatures  not  imperfect 
because  thus  small,  but  endowed  with  organs  and  parts  as  com- 
plex and  curious  as  those  of  an  elephant.  And  he  who  ad- 
mires the  curious  structure  of  a  honey  comb,  may  bend  his 
eye  through  the  microscope  upon  the  cut  surface  of  a  willow- 
branch,  or  of  other  wood,  there  to  see  a  similar  structure  more 
wonderful  still:  or  he  may  compare  the  lace  of  a  fly's  wing  with 
the  most  perfect  which  human  art  can  weave;  or  the  beautiful 
proportions  and  perfection  of  the  limbs  and  weapons  of  an  in- 
sect, invisible  perhaps  to  the  naked  eye,  with  any  larger  ob- 
jects of  the  kind  already  known  to  him. 

Telescopes  and  microscopes  might,  with  propriety?  be  both 
called  microscopes,  for  often  the  telescopic  object  appears  to 
the  naked  eye  even  smaller  than  that  which  the  microscope  ex- 
amines. The  minutest  visible  insect  at  hand  may  hide  from 
the  eye  a  planet  at  a  distance.  The  image  in  the  telescope  is 
smaller  than  in  the  microscope,  because  the  rays  from  a  dis- 
tance being  nearly  parallel,  must  form  the  image  nearly  in  the 
focus  of  the  object  glass;  while  for  the  microscope,  the  rays 
from  the  near  object  being  very  divergent,  form  the  image  far 
beyond  'the  focus,  and  praport 


"  Light  falling  on  very  smooth  or  polished  and  flat  sur- 
faces is  reflected  so  nearly  in  the  order  in  which  it  falls, 

as  to  appear  to  the  eye  as  if  coming  direc  fly  from  the  ob- 
jects originally  emitting  it,  and  such  surfaces  are  called 

MIRRORS."     (Read  the  Analysis,  page  122.) 

If  a  marble  slab,  or  other  flat  surface. 
were  in  the  situation  M  R,  with  its 
edges  towards  the  spectator,  a  ball  pro- 
jected from  A  perpendicularly  towards 
it  at  D,  would  rebound  directly  back  to 
A,  but  if  projected  from  it  obliquely, 
as  from  B  to  D,  it  would  not  return  to 
the  first  situation  B,  but  to  b,  a  situa- 
tion as  distant  on  the  opposite  side  of 
the  perpendicular;  thus  making  the  angle  of  the  return  or  re- 


MIBKOKS 


flection  equal  to  the  angle  of  approach  or  incidents  :  the  same 
would  be  true  of  a  ball  approaching  obliquely  from  any  other 
point,  as  C,  and  returning  to  c.  Now,  light  is  reflected  from 
polished  surfaces  according  to  the  same  law,  so  that  an  eye  at 
A  would  see  itself  as  if  placed  at  d,  an  eye  at  b  would  see  an 
object  really  at  B  as  if  it  were  at  e,  and  so  forth.  Where  the 
existence  of  a  mirror  is  not  suspected,  the  objects  reflected 
from  it  are  held  to  be  realities  placed  beyond  where  it  is.  A 
wild  animal  will  attack  its  image  in  a  glass;  and  a  dog  crossing 
a  brook,  will  quit  the  piece  of  meat  in  its  mouth  to  catch  the 
tempting  image  which  the  eye  sees  in  the  water  below.  The 
reason  that  an  object  seen  in  a  plane  mirror  appears  to  be  just 
as  far  beyond  the  mirror  as  its  true  distance  on  the  side  of  the 
spectator,  is,  that  the  diverging  rays  of  a  pencil  of  light  have 
the  same  divergence  after  as  before  reflection. 

Any  plane  very  smooth  surface  reflects  light  as  now  de- 
scribed, and  is  a  mirror;  but  different  substances  send  back 
very  different  proportions  of  the  light  which  falls  on  them.  — 
A  highly  polishp.d  mptaHin.  surfapp  is  thp  hest  mirror,  often  re- 
turning three-fourths  of  the  whole  light.  Hence,  in  reflecting 
telescopes,  the  mirrors  are  made  of  polished  metal. 

Our  common  looking-glasses  are  really  metallic  mirrors,  for 
it  is  the  smooth  clear  surface  of  the  quicksilvered  tin  foil  be- 
hind the  glass  which  reflects  the  light,  the  glass  itself  merely 
serving  the  purpose  of  preserving  the  metallic  surface  perfect- 
ly clear  and  flat.  There  is  always  an  imperfection  in  such 
glass  mirrors,  when  used  for  viewing  oblique  objects,  because 
the  glass  bends  the  light  a  little,  and  because  the  external  sur- 
face of  the  glass,  acting  also  as  a  mirror,  although  so  much 
more  feebly  than  the  metal  behind,  forms  an  image  which 
mixes  with  and  confuses  the  other. 

The  mirror  power  of  glass,  unaided,  is  seen  from  the  panes 
of  a  plate-glass  window,  which  make  objects  in  front  very  vi 
sible,  although  by  no  means  with  clearness  comparable  to  that 
from  a  metallic  surface.  All  common  panes  of  glass  in  win- 
dows, or  covering  print-frames,  &c.,  reflect  as  much  light  as 
plate  glass,  but  the  reflection  being  irregular  because  the  sur- 
face is  irregular,  it  does  not  attract  notice. 

The  smooth  surface  of  a  fluid  is  a  mirror,  and  which  more- 


220  LIGHT. 

over  is  horizontal;  and  when  that  surface  is  metallic,  as  of  mer- 
cury, the  mirror  is  most  perfect.  In  water,  spirits,  oil  or  any 
other  liquid,  it  is  also  perfect,  but  feebler. 

The  mirror  of  liquid  quicksilver  is  sometimes  used  by  as- 
tronomers in  observing  the  apparent  altitudes  of  the  heavenly 
bodies;  for  the  image  in  the  mirror  appearing  exactly  as  much 
below  the  horizon  as  the  object  is  really  above  it,  half  the  dis- 
tance between  them  is  the  true  height. 

A  varnished  picture,  or  any  japanned  surface,  is  a  mirror;, 
nay,  also,  even  a  polished  piece  of  wood,  for  instance,  a  ma- 
hogany table, — as  is  well  known  among  playful  children.  The 
author,  while  writing  this,  is  looking  on  a  table  covered  with 
black  leather,  and  in  that  covering,  as  a  mirror,  he  clearly  sees 
all  bright  objects  beyond  the  table.  Polished  stones,  as  mar- 
ble slabs,  &c.,  reflect  as  much  as  glass.  But  even  a  surface  of 
air  may  be  a  mirror,  as  where  a  cold  and  dense  stratum  hap- 
pens to  be  in  contact  with  a  warmer  and  rarer  stratum;  hence, 
the  trees,  islands,  &c.,  reflected  from  below,  and  seen  in  the 
sky  where  particular  causes  have  unequally  heated  different 
levels  of  the  atmosphere.  On  the  burning  sands  of  Africa  a 
stratum  of  air  near  the  surface  being  heated,  that  above  some- 
times becomes  a  mirror,  thus  reflecting  more  remote  objects: 
certain  kinds  of  mist  and  thin  clouds  will  elsewhere  produce 
a  similar  effect,  so  that  in  them  a  ship  may  be  seen  as  if  sus- 
pended aloft,  with  keel  uppermost. 

An  object  seen  by  the  light  reflected  from  a  mirror  appears 
always  reversed,  as,  for  instance,  when  the  right  hand  of  a  per- 
son standing  before  a  glass  becomes  the  type  for  the  left  hand 
of  the  image — and  it  may  be  a  stump:  or  when  a  tree  or  rock, 
or  mountain,  seen  in  the  mirror  of  a  lake,  has  its  top  down- 
wards. 

It  is  on  this  account,  that  a  man  painting  his  own  portrait 
from  a  mirror,  is  apt  to  reverse  all  accidental  characteristics  of 
the  countenance  or  person,  unless  they  are  the  same  on  both 
sides;  and  then  if,  as  is  generally  true,  one  eye  be  higher  than 
the  other,  or  the  nose  be  a  little  to  one  side,  a  very  incorrect 
resemblance  will  be  produced.'  Hence,  a  person  with  a  coun- 
tenance at  all  thus  peculiar,  never  sees  himself  in  a  mirror  as 
he  appears  to  others:  and  a  belle  or  beau,  who  has  decided  that 


CAMERA  LUCIDA.  2QI 

a  curl  is  mote  graceful  on  the  left  temple,  may  unconsciously 
leave  it  on  the  right. 

By  reflecting  any  image,  however,  from  a  first  mirror  to  a 
second,  and  from  that  to  the  eye,  persons  may  see  the  object, 
or  themselves,  if  they  choose,  as  others  see  them.  What  a 
pity  that  there  are  not  some  moral  mirrors  to  answer  an  ana- 
logous purpose,  and  occasionally  to  tell  persons  how  hideous 
they  are  to  virtue's  eye,  although  they  think  of  themselves 
with  such  complacency. 

A  candle  placed  between  two  parallel  mirrors,  makes  visible 
in  either  glass  to  a  spectator  on  one  side  an  endless  straight  line 
of  lights.  If  the  glasses  be  incline-!  to  each  other,  the  lights 
will  appear  as  if  in  the  circumference  of  a  circle,  having  its  cen- 
tre where  the  prolonged  mirrors  would  meet:  this  fact'  is  well 
illustrated  in  the  beautiful  toy  called  the  kaleidoscope.  By 
placing  several  mirrors  in  particular  situations  around  an  apart- 
ment, a  man  entering  it  may  see  himself  multiplied  into  a*  crowd; 
and  a  few  ornamental  pillars  may  produce  the  effect  of  thousands 
formed  into  long  colonnades  or  retiring  lines. 

The  sun  or  moon  reflected  in  a  still  lake,  appear  as  they  do 
in  the  sky;  but  if  the  surface  of  the  water  become  at  all  ruffled 
by  the  breeze,  instead  of  one  distinct  image,  there  will  be  a  long 
line  of  bright  tremulous  reflection.  The  reason  of  this  appear- 
ance is,  that  every  little  wave,  in  an  extent  perhaps  of  miles, 
has  some  part  of  its  rounded  surface  with  the  direction  or  ob- 
liquity which,  according  to  the  required  relation  of  the  an- 
gles of  incidence  and  reflection,  fits  it  to  reflect  the  light  to  the 
eye,  and  hence  every  wave  ,in  that  extent  sends  its  momen- 
tary gleam,  which  is  succeeded  by  others. 

Although  the  external  surface  of  glass  does  not  reflect  but 
a  small  part  of  the  light  which  falls  upon  it,  being  therefore  a 
feeble  mirror,  very  curiously,  if  light,  which  has  entered-  a 
piece  of  glass,  fall  upon  a  back  or  internal  surface,  very  oblique- 
ly, instead  of  passing  out  there,  it  is  more  perfectly  reflected 
than  it  would  be  by  the  best  metallic  mirror.  Thus  light  from 
A  entering  at  B,  is  entirely  reflected  at 
C,  and  escapes  at  D  towards  E.  The 
back  of  a  wedge  of  glass,  or  common 
prism,  thus  becomes  a  perfect  mirror. 
It  is  this  fact  which  enabled  Dr.  Wol- 


222  LIGHT. 

laston  to  construct  that  beautiful  little  instrument  called  by  bim 
the  Camera  Lucida.  The  two  surfaces 
at  the  back  of  the  small  prism  of  glass  A 
become  mirrors,  the  first  reflecting  to  the 
IC—  second,  and  the  second  to  the  eye  at  E, 
the  objects  in  the  landscape  before  it  while 
the  eye  also  sees  through  the  glass  to  the 
paper  below  at  B,  and  may  suppose  the 


imagery  to  be  feebly  portrayed  on  the  paper:  with  a  pen- 
cil that  appearance  is  made  permanent,  and  a  correctly-drawn 
outline  of  the  scene  is  at  once  obtained.  This  instrument  for 
assisting  draftsman  is  still  simpler  than  the  camera  obscura. 
Other  modifications  of  the  instrument  have  since  been  contrived. 

The  same  fact  of  the  back  and  internal  surface  of  a  transparent 
mass  becoming  a  mirror,  gives  us  the  explanation  of  that  phe- 
nomenon so  admired  before  it  was  understood,  and  not  less  ad- 
mired since,  viz.  the  rainbow,  or  arc  in  the  sky,  as  in  France 
and  elsewhere  it  is  named — the  object  which  the  poets  of  na- 
ture have  almost  worshipped  for  its  beauty,  one  of  the  de- 
lights of  our  boyish  days,  when  we  saw  it  stretching  over  the 
haunts  of  our  young  pleasures,  and  when  we  pursued  it  in  the 
hope  of  catching  some  of  the  falling  rubies  and  emeralds,  or 
bright-coloured  dew  of  which  it  might  be  composed. 

When  a  partial  shower  of  rain  falls  on  the  side  of  the  land- 
scape opposite  to  where  the  sun  is  shining,  this  beautiful  phe- 
nomenon immediately  appears,  viz.  a  variegated  arch,  red  at 
its  external  border- or  confine,  and  then  successively  orange, 
j^ellow,  green,  &c.  (in  the  order  of  the  colours  of  the  prismatic 
spectrum  described  at  page  142,)  to  wards  its  inner  border  or  con- 
fine: its  centre  is  directly  opposite  to  the  sun,  as  if  at  the  end  of  a 
straight  line  drawn  from  the  sun  through  the  eye  of  the  specta- 
tor towards  the  opposite  horizon,  and  being  therefore  always  un- 
der the  horizon,  the  bow  is  less  than  a  semicircle.  The  dia- 
ameter  of  the  circle  of  which  the  bow  is  a  part  is  of  about  82°  of 
the  field  of  view.  There  is  a  second  bow  of  much  fainter  light 
than  the  first,  and  with  the  colours  in  reverse  order:  it  is  of 
108°  diameter,  and  therefore  external  to  the  other. 

Now,  the  explanation  of  this  miracle  of  beauty  is  simply  as 
follows.  While  the  sun  shines  upon  the  spherical  drops  of  fall- 
ing rain,  and  its  light  falling  upon  the  whole  central  part  of  any 


HAISBOW. 


drop,  passes  completely  through,  still  that  portion  which  enters 
near  the  edge  of  the  drop,  as  at  «,  is  refracted,  and  reaches  the 
back  surface  of  the  drop  at  y  so  slantingly,  or  at  an  angle  so 
great,  that  there  occurs  an  entire  reflection 
of  it  instead  o,f  transmission;  the  ray  therefore 
is  returned  to  b,  where  it  again  escapes  from 
the  drop,  and  as  here  shown,  descends  to  the 
earth  or  eye  at  e.  Thus  every  little  drop  of 
rain  on  which  the  sun  shines  is  a  little  mir- 
ror suspended  in  the  sky,  and  is  returning  at 
a  certain  angle  all  round  it,  viz  at  an  angle  of  41°,  a  portion 
of  the  light  which  falls  on  it;  and  an  eye  placed  in  the  required 
direction  receives  that  reflected  light.  If  in  this  case  there  were 
reflection  only,  and  not  also  refraction  with  separation  of  co- 
lours* the  rainhow  would  be  only  a  very  narrow  resplendent  arc 
of  white  light,  built  up  of  millions  of  little  images  of  the  sun; 
but  in  truth,  because  the  light  which  enters  near  the  edge  of  the 
drop  traverses  the  surface  very  obliquely,  it  is  much  bent  or  re- 
fracted before  its  reflection,  as  seen  at  a,  and  is  divided  into 
rays  of  its  seven  colours,  as  it  would  be  on  passing  through  a 
prism  (as  explained  at  page  143;)  and  this  division  or  separation 
continuing  after  the  light  again  escapes  from  the  drop  at  b,  in- 
stead of  one  white  ray  descending  from  each  drop  to  a  'certain 

point  of  the  earth,  seven  rays  de- 
scend (here  marked  by  dotted  lines 
from  the  figure  I  on-the  left  hand, 
to  7,  6,  5,  &c.,  on  the  right,)  and 
of  these  an  eye  can  only  catch  one 
at  a  time:  but  for  the  same  reason 
*l*  that  seven  eyes  placed  in  a  line 
from  above  downwards  (viewing 

the  centre  of  the  bow)  would  be  required  to  see  the  seven  co- 
lours from  one  drop,  so  one  eye  looking  in  the  direction  of 
seven  drops  situated  in  a  corresponding  line,  as  from  1  to  7, 
will  catch  the  lower  or  red  ray  of  the  upper,  the  orange  or  se- 
cond ray  of  the  next,  the  yellow  or  third  ray  of  that  which  fol- 
lows, and  so  on,  while  it  will  lose  all  the  others,  and  thus  will 
see  the  several  drops  as  if  they  were  each  of  one  colour  only. 
Of  such  elements,  then,  found  in  the  same  relative  directions 
all  around  the  eye,  the  glorious  arch  is  formed.  No  two  eyes 


224  LIGHT. 

can  see  the  same  rainbow,  that  is,  can  receive  light  from  the 
same  drops  at  the  same  time;  and  the  same  eye  does  not  for 
two  instants  receive  light  from  the  same  drops.  This  rainbow 
can  never  appear  to  a  person  on  a  plain,  unless  when  the  sun  is 
within  41°  of  the  horizon,  for  otherwise  the  centre  of  the  rain- 
bow would  be  more  than  41°  under  the  horizon,  and  41°  is  the 
whole  semidiameter  of  the  bow. 

We  have  described  above  what  is  called  the  principal  bow, 
formed  in  the  drops  by  t\vo  refractions  and  one  reflection  of 
light.  To  produce  the  fainter  second  or  external  bow,  men- 
tioned above,  and  of  which  the  colours  are  in  reverse  order, 
the  light  which  enters  at  a  is  reflected 
first  at  y,  then  again  at  b,  and  escapes 
at  c  towards  thp  eye:  hence  there  are 
'Or  Sw  two.  reflections  as  well  as  two  refractions. 

As  the  semidiameter  of  this  bow  is  54°? 
it  may  be  visible  when  the  internal  bow  is  not. 

An  artificial  rainbow  may  be  produced  in  sunshine  at  any 
time  by  scattering  water  drops  from  a  brush  or  otherwise.  The 
cut-glass  ornaments  of  chandeliers,  &c.,  produce  colours  on  the 
same  principle  as  rain  drops;  as  do  also  mist  and  particles  of 
frozen  water  between  a  luminous  body  and  the  eye,  exhibiting 
the  circular  Aa/os  often  observed  around  the  sun  and  moon. 

e(  ,ZI/m'0r,y  may  be  plane,  convex,  or  concave;  and  certain 
curvatures  will  produce  images  by  reflection*  just  as  lenses 
produce  images  by  refraction;  so  that  there  are  reflecting 
telescopes,  microscopes,  fyc.,  as  there  are  refracting  instru- 
ments of  the  same  names."  (See  the  Analysis,  page  122.) 

While  a  plane  surface  reflects  light,  so  that  what  is  called  the 
image  in  it  of  a  known  object  may  readily  be  mistaken  for  the 
reality,  convex  or  concave  mirrors  reflect  as  if  every  distinct 
point  of  them  were  a  separate  small  plane  mirror,  and  their  effects 
on  light  correspond  with  the  relative  inclination  of  the  different 
parts.  The  only  forms  of  much  importance  are  the  regularly 
spherical  or  parabolic  concave  and  convex  mirrors.  We  shall 
now  find  that  these  have  on  light  similar  effects  with  lenses, 
only  the  concave  mirror  answers  to  the  convex  lens,  and  the 
convex  mirror  to  the  concave  lens.  It  is  the  concave  mirror 
which  gathers  the  light  to  form  images  in  the  most  perfect  te- 


CURVED  MIRRORS. 

lescopes  that  exist,  as  those  of  Herschell  and  others.  Admira- 
ble as  in  certain  respects  is  the  refracting  telescope,  it  falls  much 
short  of  the  telescope  acting  by  reflection. 

In  a  holiow  sphere,  or  part  of  a  sphere  with  polished  internal 
surface,  if  rays  radiate  from  the  centre  in  all  directions,  they 

•  reach  every  part  perpendicularly,  and 
therefore  are  thrown  back  to  the  centre. 
C  Thus,  if  A  B  were  a  concave  spherical 
mirror,  of  which  C  were  the  centre,  rays 
issuing  from  C  would  again  meet  there. 

It  can  be  proved  also,  that  any  ray 
parallel  to  the  axis,  falling  upon  such 
a  mirror,  will  be  reflected  inwards 

— — /^ 

j  so  as  to  cut  the  axis  half  way  be- 
tween the  mirror  and  its  centre,  viz. 
at  D.  Then,  as  all  parallel  rays  meet  in  the  same  point,  that 
point  becomes  a  focus,  as  already  explained  for  lenses; — about 
that  part  an  image  of  the  sun,  for  instance,  will  be  formed 
when  the  mirror  is  held  directly  towards  the  sun.  This  point 
is  called  the  principal  focus  of  the  mirror. 

For  the  same  reason  that  parallel  rays  meet  in  the  focus,  so 
will  rays,  issuing  from  the  focus,  become  parallel,  after  reflec- 
tion, as  seen  in  the  figure  at  page  50;  and  if  they  be  then  caught 
in  a  second  and  opposite  mirror,  as  there  also  represented,  cor- 
responding effects  will  follow. 

Now,  for  a  concave  mirror,  as  already  explained  for  a  lens, 
when  rays  fall  on  it  obliquely  from  one  side  of  the  axis,  their 
focus  will  be  on  the  opposite  side,  and  therefore  the  mirror  will 
form  an  inverted  image  of  any  body  placed  before  it,  just  as  a 
lens  does;  and  the  image  will  be  near  or  distant,  and  large  or 
small,  according  to  the  divergence  of  the  approaching  rays,  ex- 
actly as  happens  with  lenses:  and  thus,  the  camera  obscura,  ma- 
gic lantern,  telescopes  and  microscopes  may  all  be  formed  by 
mirrors,  as  they  may  be  by  lenses.  Moreover,  concave  mirrors 
magnify,  and  convex  mirrors  minify,  as  concave  lenses  of  the 
opposite  names  do.  The  two  subjects  of  images  by  refraction 
and  by  reflection  run  so  nearly  parallel,  that  it  would  be  useless 
repetition  here  to  enter  upon  the  detailed  consideration  of  the 
htter  subjectj  and  we  shall  therefore  content  ourselves  with  sffbw- 


Liulii 


"=" 


i. 

ib 


ing  why  a  concave  mirror  magnifies,  and  why  a  convex  mirror 
minifies. 

A  concave  mirror  magni- 

C ...  "••-...    ^  A     fies,  because  the  .light  from 

A  reaching  the  mirror  where, 
it  can  be  reflected  to  an  eye 
placed  at  F.  viz.  at  E,  seems 
to  the  eye  to  come  from  C, 

and  the  light  of  B  similarly  appears  to  come  from  D,  so  that  the 
cross  A  B,  by  the  reflection,  seems  to  the  eye  to  be  of  the  greater 
dimensions  C  D. 

In  a  convex  mirror,  again,  for  cor- 
responding reasons,  the  cross  A  B  ap- 
pears only  asC  D,  and  therefore  much 
smaller  than  the  reality. 

Concave,  or  magnifying  mirrors,  are 
often  used  by  persons  in  shaving. 
A  convex  mirror  is  a  common   ornament  of  our  apartments, 
exhibiting  a  pleasing  miniature  of  the  room  and  its  contents. 

Any  polished  convex  body  is  a  mirror,  and  therefore  the  bail 
of  the  human  eye  is  one  in  whioh  we  may  contemplate  most 
perfect  miniatures  of  surrounding  tilings.  It  is  the  image  of 
the  window,  or  of  the  sun,  in  the  convex  mirror  of  the  eye, 
which  painters  usually  represent  by  a  spot  of  white  paint  there; 
and  a  similar  luminous  spot  or  line  must  be  made  when  they  re- 
present almost  any  of  the  pieces  of  furniture  which  have  round- 
ed polished  surfaces,  a%s  bottles,  glasses,  smooth  pillars,  &c. 

Convex  lenses  thus  are  mirrors  to  all  the  objects  around  them, 
and  very  strikingly  so,  owing  to  the  perfection  of  the  form  of 
a  lens.  The  polished  back  of  a  watch,  often  in  the  same  way 
attracts  the  attention  of  a  child,  who  wonders  to  see  there  so 
clearly  '  a  little  baby.' 

It  has  been  a  mathematical  amusement  to  calculate  what  kind 
of  distortion  mirrors  of  unusual  forms  will  produce,  and  then 
to  make  distorted  drawings,  which,  reflected  from  such  mirrors, 
might  produce  in  the  eye  the  natural  image  of  the  objects. 

When  a  concave  mirror  is  used  for  a  telescope,  the  image 
formed  in  front  of  it,  and  examined  usually  through  a  powerful 
magnifying  eye  glass,  may  be  viewed, — as  in  HerschelFs  tele- 
scope, by  the  spectator  turning  his  back  to  the  real  object,  and 


REFLECTING   TELESCOPES.  227 

looking  in  at  the  mouth  of  the  telescopic  tube,  near  to  the  edge 
of  which  the  image  is  thrown  by  a  slight  inclination  of  the  mir- 
ror at  its  bottom: — or  as  in  the' <\eivtonian  telescope,  through 
an  opening  in  the  side  of  the  tube,  after  being  reflected  by  a 
small  plane  mirror  placed  diagonally  in  the  centre  of  the  tube: 
— or  as  in  the  Gregorian  telescope,  through  an  opening  cut  in 
the  principal  mirror  or  speculum,  after  being  reflected  towards 
that  opening  by  a  smaller  mirror  placed  in  the  centre  of  the  tube: 
this  last  arrangement  is  that  preferred  for  smaller  telescopes,  be- 
cause the  spectator,  while  seeing  the  image,  is  also  looking  in 
the  direction  of  the  object. 

Reflecting  telescopes  have  the  advantage  of  being  perfectly 
achromatic,  that  is,  of  producing  no  coloured  or  rainbow  edges 
to  the  images:  for  compound  light  is  reflected,  although  not  re- 
fracted, entire,  all  the  colours  following  the  same  law  of  equal 
angles  of  incidence  and  reflection. 

HerschelPs  largest  telescope  had  a  mirror  of  48  inches  in 
diameter,  and  therefore  received  about  150,000  times  more 
light  than  an  unassisted  eye  could,  making  with  it,  at  a  focal 
distance  of  40  feet,  a  large  image  admirably  distinct.  It  was 
with  this  that,  in  the  obscurity  of  remote  space,  he  discovered 
rolling  along  the  immense  planet,  which,  in  honour  of  his 
royal  patron,  he  called  the  Georgium  Sidus,  but  which  now, 
by  the  decision  of  the  scientific  world,  bears  his  own  name; — 
and  with  this  he  discovered  moons  before  unseen,  of  other 
planets,  and  he  unravelled  the  celestial  nebulae  and  clustered 
stars  of  the  milky  way,  and,  in  a  word,  unveiled,  vastly  more 
than  had  before  been  done,  the  system  of  the  boundless  uni- 
verse. If  this  world  were  to  last  for  millions  of  years,  the 
discoveries  of  HerschelPs  telescope  would  mark  a  memorable 
epoch  of  its  early  history. 

"  Light  returned  from,  or  passing  through  bodies  of 
rougher  or  irregular  surface,  or  which  have  other  pecu- 
liarities, is  so  modified  as  to  produce  all  those  phenome- 
na of  colour  and  varied  brightness  seen  among  natural 
bodies*  and  giving  them  their  distinctive  characters  and 
beauty."  (See  the  Analysis,  page  122.) 

General  remarks  on  this  part  of  our  subject  were  made  in 


J-JS  LIGHT. 

the  beginning  of  the  section,  in  the  explanations  of  how  ob- 
jects, riot  self-luminous,  become  visible  by  reflecting  the  light 
of  other  bodies,  and  of  how  the  prism  separates  a 'ray  of  white 
light  into  rays  of  the  several  colours  of  the  rainbow — which 
rays,  on  being  again  mixed,  reproduce  white  light  as  before: — 
and  much  beyond  these  remarks  we  have  not  the  intention  of 
now  proceeding.  To  give  a  full  account  of  the  matters  that 
come  within  the  scope  of  this  depaitment,  would  occupy  the 
pages  of  a  large  volume;  for  there  would  be  to  pass  in  review 
the  various  opinions  which  have  existed  on  the  intimate  na- 
ture, of  light, — the  facts  connected  with  what  has  been  called 
the  polarisation  of  light, — the  relation  of  light  in  double  re- 
fraction, to  the  ultimate  structure  of  material  masses,  &c  ,  all 
which  subjects  are  in  certain  respects  highly  interesting;  but — 
as  some  of  them  are  not  yet  completely  investigated — as  re- 
specting others,  various  opinions  prevail, — as  they  involve  few 
matters  applicable  to  common  use, — as  the  reasonings  about 
them  are  far  removed  from  ordinary  trains  of  thinking,  and 
refer  to  facts  altogether  unknown  to  common  observation  — 
we  hold  them  not  to  be  fit  parts  of  a  popular  treatise  on  light. 
We  may  state,  however,  that  persons  who  have  the  leisure  and 
the  mathematical  preparation  necessary  for  pursuing  the  study, 
will  find  their  labour  in  it  richly  rewarded. 

What  we  deem  it  necessary  then  here  to  aJd  is,  that  white 
light,  in  falling  upon  any  transparent  substance,  as  air,  water, 
glass,  &c.,  reduced  to  thin  plates  or  films,  is  so  affected,  that 
for  certain  degrees  of  thinness,  different  for  each  substance, 
it  is  decomposed,  and  is  reflected  or  transmitted  not  as  white 
light,  but  as  some  of  the  colours  of  the  rainbow,  and  the  co- 
lour reflected  in  any  case  is  always  the  opposite  or  complement 
of  that  which  passes  through,  that  is  to  say,  such  that  the  two 
brought  together  again  make  white  light.  The  facts  may  be 
studied,  as  Newton  originally  studied  them,  in  the  thin  plate 
of  air  which  occupies  the  space  between  a  convex  lens  and  a 
plane  surface  of  glass  upon  which  the  lens  is  laid, — in  which 
plate,  as  the  distance  from  the  point  of  the  apparent  contact  of 
the  glasses  increases,  there  appear  successive  rings  of  vivid  co- 
lours. The  same  truth  is  exemplified  in  the  colours  of  a  soap 
bubble,  which  brighten  as  the  bubble  swells  and  becomes  of 


COMPARISON  OF  LIliHT  AND  SOUND.  JJJ>H 

thinner  substance,  and  are  different  as  the  thickness  increases 
from  above  downwards; — and  it  is  exemplified  in  numerous 
other  common  facts.  Now,  whatever'  be  the  reasons  of  such 
decomposition  of  light — and  the  explanation  is  not  yet  com- 
plete— we  cannot  doubt  that  in  natural  bodies  generally,  the 
colours,  opacity,  transparency,  &.C.,  depend  entirely  upon  the 
volume  and  arrangement  of  the  minute  fibres  or  plates,  with  in- 
cluded interstices,  which  constitute  the  volume  or  structure  of 
each  mass.  Accordingly,  whatever  changes  that  arrangement, 
may  change  also  the  colour  of  the  mass.  Thus,  by  drawing  a 
certain  number  of  minute  lines  on  a  certain  extent  of  any  me- 
tallic surface,  we  may  make  it  of  what  colour  we  please;  and 
mother-of-pearl  owes  its  beauty  entirely  to  its  furrowed  or  stri- 
ated surface,  as  is  proved  by  our  taking  an  impression  of  that 
surface  on  sealing-wax,  and  perceiving  that  the  wax  then  exhi- 
bits similar  colours. 

The  investigations  in  progress  respecting  the  phenomena  of 
light,  are  furnishing  new  proofs  of  the  extreme  simplicity  of 
nature,  amidst  the  boundless  extent  and  infinite  variety.  When 
men  thought  of  the  sense  of  touch  only  as  it  exists  at  the  tips 
of  the  fingers,  or  on  the  general  surface  of  the  body,  they 
were  far  from  suspecting  that  the  sense  of  hearing  had  the  near 
relation  to  it  which  subsequent  discoveries  have  proved,  and 
still  less  did  they  think,  that  the  sense  of  sight  was  similarly 
related;  but  step  by  step  they  ascertained,  1st,  of  sound  coming 
to  the  ear  through  the  air — that  air  was  a  material  fluid  as  much 
as  water,  consisting  of  the  same  or  similar  particles,  only  more 
distant  among  themselves — that  a  motion  or  trembling  in  the 
air,  by  affecting  nerves  exposed  in  the  ear,  produced  the  sensa- 
tion of  sound,  as  the  trembling  in  a  log  of  wood  caused  by  the 
action  of  a  saw  produces  a  peculiar  sensation  of  touch  in  a  hand 
laid  on  the  log, — and,  finally,  that  common  sound  in  all  its  va- 
rieties, is  merely  such  trembling  of  the  air,  affecting  a  struc- 
ture of  nerve  so  exposed  in  the  ear,  as  to  be  as  much  more  rea- 
dily excitable  than  the  nerves  in  the  fingers  and  elsewhere  in 
the  skin,  as  the  action  or  impulse  of  moving  air  is  more  deli- 
cate than  that  of  common  solids  and  liquids.  And  now,  in 
the  investigations  respecting  light,  this  kind  of  comparison  is 
carried  a  step  farther;  for  it  is  become  matter  almost  of  certain- 


230  LIGHT. 

ty  that  the  sensation  of  light  is  produced  in  a  suitable  nervous 
tissue  in  the  eye,  by  a  trembling  motion  in  another  fluid  than, 
air,  which  fluid  pervades  all  space,  and  in  rarity  or  subtlety  of 
nature  surpasses  air  vastly  more  than  air  does  water  or  solids; 
and  while  in  sound,  different  tones  or  notes  depend  on  the 
number  of  vibrations  in  a  given  time,  so  in  light  do  different 
colours  depend  on  the  extent  of  the  single  vibrations.  Can 
human  imagination  picture  to  itself  a  simplicity  more  magni- 
ficent and  fruitful  of  marvellous  beauty  and  utility  than  this! 
But,  farther,  as  air  answers  in  the  universe  so  many  important 
purposes  besides  that  of  conveying  sounds,  although  this  alone 
comprehends  language,  which  almost  means  reason  and  civili- 
zation— so  also  does  the  material  of  light  minister  in  numerous 
ways,  in  the  phenomena  of  heat,  electricity,  and  magnetism. 

The  truths  now  positively  ascertained  with  respect  to  the 
nature  of  light  and  vision,  are  perhaps  those  in  the  wide  field 
of  human  inquiry  which,  acting  on  ordinary  apprehension, 
most  forcibly  place,the  individual  as  it  were  in  the  presence  of 
Creative  Intelligence,  and  awaken  the  most  .elevated  thoughts 
of  which  the  human  mind  is  capable.  Had  there  been  no 
light  in  the  universe,  all  its  other  perfections  had  existed  in 
vain.  Men  placed  on  earth  would  have  been  as  human 
exiles  with  their  eyes  put  out,  abandoned  on  an  unknown 
shore,  of  climate  and  productions  totally  new  to  them:  every 
movement  might  be  to  destruction,  for  their  perceptions  would 
be  limited  by  the  length  of  their  arms,  and  of  their  fearful 
groping  steps,  and  the  wretched  beings,  separating  when  im- 
pelled by  hunger  to  search  for  food,  would  probably  scatter  to 
meet  no  more.  But  the  material  of  light  exists,  pervading  all 
space,  and  certain  impressions  made  upon  it  in  one  place  ra- 
pidly spread  over  the  universe,  the  progressive  impression  be- 
ing called  a  ray,  or  beam  of  light.  The  beams  of  light,  then, 
from  all  parts  coming  to  every  individual,  may  be  regarded  as 
supplementary  arms  or  feelers  belonging  to  the  individual,  and 
which  reach  to  the  end  of  the  universe,  so  that  each  person, 
instead  of  being  as  a  blind  point  in  space,  becomes  nearly  om- 
nipresent:— then  these  limbs  or  feelers  have  no  weight,  they 
are  never  in  the  way,  they  impede  nothing,  and  they  are  only 
known  to  exist  when  their  use  is  required!  But  this  miracle  of 


PERFECTION  Otf  THE  L\L. 


light  would  have  been  totally  useless,  and  the  lovely  paradise 
of  earth  would  have  been  to  man  still  a  dark  and  dreary  de- 
sert, had  there  not  been  the  twin  miracle  of  an  organ  of  com- 
mensurate delicacy  to  perceive  the  light,  viz  of  the  Eye;  — 
in  which  there  is  the  round  cornea  of  such  perfect  transparence, 
placed  exactly  in  the  anterior  centre  of  the  ball,  (and  elsewhere 
it  had  been  useless,)  then  exactly  behind  this,  the  beautiful  cur- 
tain the  iris,  with  its  pupil  dilating  and  contracting  to  suit  the 
intensity  of  light  —  and  exactly  behind  this  again,  the  crystal- 
line lens,  having  many  qua'ities  which  only  complex  structure 
in  human  art  can  attain,  and  by  the  entering  light  forming  on 
the  retina  beautiful  pictures  or  images  of  the  objects  in  front, 
the  most  sensible  part  of  the  retina  being  where  the  images 
fall..  Of  these  parts  and  conditions,  had  any  one  been  other- 
wise than  as  it  is,  the  whole  eye  had  been  useless,  and  light  use- 
less, and  the  great  universe  useless  to  man,  for  he  could  not  have 
existed  in  it.  Then,  farther,  we  find  that  the  precious  organ, 
the  eye,  is  placed,  not  as  if  by  accident,  somewhere  near  the 
centre  of  the  person,  but  aloft  on  a  proud  eminence,  where  it 
becomes  the  glorious  watch-tower  of  the  soul;  and,  again,  not 
so  that  to  alter  its  direction  the  whole  person  must  turn,  but 
in  the  head,  which  on  a  pivot  of  admirable  structure  moves 
while  the  body  is  at  rest;  the  ball  of  the  eye,  moreover,  being 
furnished  with  muscles  which,  as  the  will  directs,  turn  it  with 
the  rapidity  of  lightning  to  sweep  round  the  horizon  or  take 
in  the  whole  heavenly  concave;  —  then  is  the  delicate  orb  se- 
cured in  a  strong  socket  of  bone,  and  there  is  over  this  the 
arched  eye-brow  as  a  cushion  to  destroy  the  shock  of  blows, 
and  with  its  inclined  hairs  to  turn  aside  the  descending  perspi- 
ration which  might  incommode;  —  then  is  there  the  soft  and 
pliant  eye-lid  with  its  beauteous  fringes,  incessantly  wiping 
the  polished  surface,  and  spreading  over  it  the  pure  moisture 
poured  out  by  the  lachrymal  glands  above,  of  which  moisture 
the  superfluity  by  a  fine  mechanism  is  sent  into  the  nose,  there 
to  be  evaporated  by  the  current  of  the  breath:  —  still  farther,  in- 
stead of  there  being  only  one  so  precious  organ,  there  are  two, 
lest  one,  by  accident,  should  be  destroyed,  but  which  two  have 
so  entire  a  sympathy,  that  they  act  together  as  only  one  more 
perfect;  —  then  the  sense  of  sight  continues  perfect  during  the 


LIfctHT. 


period  of  growth  from  birth  to  maturity,  although  the  distance 
from  the  lens  to  the  retina  is  constantly  varying;  —  and  the 
pure  liquid  which  fills  the  eye,  if  rendered  turbid  by  disease 
or  accident,  is  by  the  actions  of  life,  although  its  source  be  the 
thick  red  blood,  gradually  restored  to  transparency.  The 
mind  which  can  suppose  or  admit  that  within  any  limits  of 
time,  even  a  single  such  organ  of  vision  could  have  been  pro- 
duced by  accident,  or  without  design,  —  and  still  more,  that 
the  millions  which  now  exist  on  earth,  all  equally  perfect,  can 
have  sprung  from  accident  —  or  that  the  millions  of  millions  in 
past  ages  were  all  hut  accidents  —  and  that  the  endless  millions 
throughout  the  animate  creation,  where  each  requires  a  most 
peculiar  fitness  to  the  nature  and  circumstances  of  the  animal, 
can  be  accident  —  must  surely  be  of  extraordinary  character, 
or  must  have  received  unhappy  bias  in  its  education. 

As  a  concluding  reflection  with  respect  to  vision  we  may  re- 
mark, that  all  the  provisions  above  considered  have  mere  uti- 
lity in  view,  for  any  one  of  them  wanting  would  leave  a  ne- 
cessary link  in  the  chain  of  creation  wanting:  but  we  have 
shown  in  a  preceding  part  of  the  work,  that  if  there  had  been, 
white  light  only,  susceptible  of  different  degrees  of  intensity 
and  shade,  the  merely  useful  purposes  of  vision  would  have 
been  answered  about  as  perfectly  as  with  all  the  colours  of  the 
rainbow  —  which  truth  is  instanced  in  the  facts,  that  many  per- 
sons do  not  distinguish  colours,  and  that  it  imports  not  whe- 
ther a  person  view  objects  in  the  morning,  or  at  mid-day,  or 
at  even-tide,  or  through  plane  glass  or  coloured  glass.  While, 
therefore,  the  existence  of  light  generally,  and  of  the  eye, 
speaks  of  Creative  Power  and  Intelligence,  the  existence  of 
colours,  or  of  that  lovely  variety  of  hues  exhibited  in  flowers, 
in  the  plumage  of  birds,  in  the  endless  aspects  of  the  earth  and 
heavens,  in  a  word,  in  the  whole  resplendent  clothing  of  na- 
ture, —  because  appearing  expressly  planned,  as  a  source  of  de- 
light to  animated  beings,  speaks  of  Creative  Benevolence,  and 
may  well  excite  in  us,  towards  the  Being  in  whom  these  attri- 
butes concentrate,  the  feelings  associated  in  our  minds  during 
this  earthly  scene,  with  the  endearing  appellation  of  'Father.' 

END  OF  THE  SECTION  ON  LIGHT- 


NOV  19  1917 
APR 


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